in man and other species. These are always of the same sex. Ordinary or dissimilar twins, of course, arise from different ova and may or may not be of the same sex, as is the case in ordinary litters of young in mammals. In the nine-branded armadillo a litter contains four young, all of the same sex, and these have been conclusively shown to come from a single egg, and in a related species the polyembryonic litter contains eight or nine. In certain hymenopterous insects (chalcids) a single ovum produces a great number, in some cases hundreds of individuals. It follows from the method by which sex is determined at fertilization that all embryos thus arising from a single zygote must be of the same sex.

The Germ Cells. The great generalization on which modern embryology is based is the cell concept as applied to the gametes. This is the fact that the ovum and spermatozoön are single cells of the parent organisms, and correlated with this the relatively new knowledge of the physical basis of heredity as located in the chromosomes. It is essential to realize not only that the gametes are true cells, but that they are exactly equivalent as regards their chromatin content and consequently their heredity-carrying capacity (with the exception of the sex-chromosomes, for which see the articles CELL and HEREDITY), and that their great diversity in size and form represents only a physiological differentiation by which the spermatozoön, minute and capable of locomotion is enabled to reach the egg, which as it is supplied with foodstuff for the future embryo is much larger and non-motile. It is scarcely possible to conceive of two types of cells more widely different in form and appearance, yet both are the descendants of similar primordial germ cells, and their differences, except for the sex-chromosomes above mentioned, are entirely in the extranuclear structures. The spermatozoa are proliferated in the testis in enormous numbers. In their commonest form, often described as tadpole-shaped, there is a head composed of condensed nuclear chromatin, a middle piece containing a centrosome, and a vibratile flagellum or tail by means of which the spermatozoön is actively propelled and enabled to reach the egg. Frequently also a pointed body, the acrosome, is present at the anterior end and facilitates penetration into the ovum. There is no relation between size of spermatozoön and size of organism. In man the entire length is 52-62 thousandths of a millimeter. In many minute invertebrates it is very much greater. In a few animals, the spermatozoa are non-motile and not of the usual flagellated form. The ovum, or egg, is always much larger than the spermatozoön, non-motile and usually of spherical form. During the elaboration of the egg in the ovary granules of inert food-yolk or deutoplasm are stored up in its extranuclear protoplasm. This food-yolk is rich in protein, fts, lecithin, etc., and serves during developent as food for the embryo. The difference in size of eggs of different species is largely a difference in the amount of yolk and according to distribution of this substance eggs are described as (a) alecithal or homolecithal, having very little yolk evenly distributed as in the minute ova of mammals; (b) telolecithal, with the yolk massed toward one pole of the egg, the condition in most vertebrate eggs; and

(c) centrolecithal, in which central mass of yolk is surrounded by a superficial layer of protoplasm, a type occurring in some arthropods. The amount of yolk affects the development of the egg profoundly. The largest eggs are those of sharks, reptiles and birds, which are of extreme telolecithal type and comprise the largest cells known. Those of placental mammals are very minute, that of man only 17 hundredths of a millimeter in diameter. In oviparous animals the eggs are usually enclosed in protecting envelopes of which some are formed in the ovary and others secreted by the lining of the oviduct. In the hen's egg, to cite a familiar example, the delicate membrane surrounding the yolk is of ovarian origin, while the albumen, shell membrane and shell are oviducal secretions. Frequently, as in insects and bony fishes, the egg membrane is pierced by one or more minute pores, micropyles, which permit ingress of the spermatozoön at fertilization.

Maturation. A phenomenon long known to be of almost universal occurrence in the history of the egg is the successive extrusion from it coincident with or shortly preceding fertilization, of two minute globules known as "polar bodies." The significance of these bodies long remained a problem, the solution of which during the later years of the 19th century constituted one of the most brilliant discoveries of cellular biology. It invested chromatin with a new importance, rendered possible a new understanding of germ-cells and fertilization and opened a new avenue for the investigation of the mechanism of heredity. It is a well-established fact that the cells composing the body (somatic cells) of every animal contain a definite number of rods of chromatin called chromosomes, this number characteristic of the particular species; also that these chromosomes are in even number and composed of two equivalent groups derived respectively from the two parents (an exception to this occurs in the case of the sex chromosomes. See articles on CELL and HEREDITY). By a series of researches beginning in 1883, in connection with which the names of E. Van Beneden, Theodor Boveri and Oscar Hertwig are especially identified, it was demonstrated that the ripe germ-cells of both sexes have only one-half the somatic number of chromosomes, though in the earlier primordial germ-cells the full somatic number occurs. This reduction is accomplished through a phenomenon known as "synapsis" or union in pairs of the chromosomes of paternal and maternal origin. Thus the somatic "diploid" number of singles chromosomes becomes reduced in germcells to the "haploid" number of bivalent or double chromosomes, this reduction occurring in the spermatocyte or oöcyte cell generation prior to the last two cell divisions known as maturation divisions, by which the definitive gametic cells are formed. During the maturation divisions the bivalent chromosomes are twice divided and the resultant univalent chromosomes distributed, still in haploid_number, to each of the four resulting cells. In the male these four cells all develop into functional spermatozoa, but in the female the di visions are so unequal as to consist merely in the successive extrusion from the egg of two abortive eggs or polar bodies. In some cases the first of these bodies again divides so that

the end result is one functional egg and three polar bodies, which differ from the egg only in the smaller amount of cytoplasm and yolk, their chromatin content being exactly equivalent. The racial significance of the reduction of the number of chromosomes to one-half in both gametes will be obvious in connection with the union of these cells in fertilization. Fig. 1. Fertilization.- "Fertilization" as applied to the union of gametic cells is a somewhat inadequate term, a relic of earlier days when it was supposed that the male semen merely activated the germ contained in the egg. While it is quite true that the spermatozoön does initiate development of the egg and thus "fertilizes» it in the same sense in which artificial treatment with chemicals may fertilize many kinds of eggs, another essential fact of the conjugation of the two gametes. is the combination in the new zygote of two equivalent groups of chromosomes from the two parents. In many invertebrates and some aquatic vertebrates eggs and sperm are shed in the water, where conjugation occurs, but in many other animals the spermatozoa are transferred to the genital ducts of the female and fertilization is internal. Only one spermatozoön is normally concerned in the fertilization of an egg, though polyspermy, or the penetration of several into the egg-cytoplasm, frequently occurs, especially in forms having large eggs, but such supernumerary sperms always degenerate eventually and take no part in the formation of the embryo. When a spermatozoön comes in contact with the ovum it penetrates the cytoplasm and in many cases a delicate membrane, the fertilization membrane, is instantly secreted from the surface of the egg, thus preventing the entrance of any more sperms. At the same time other marked evidences of disturbance of the physicochemical equilibrium occur, often with violent streaming and new arrangement of formative zones in the protoplasm, and in some eggs the promorphology is rapidly established at this time. The tail, which is of no further use after the sperm has reached the egg, is frequently left outside. The head upon entrance speedily enlarges and assumes a vesicular appearance, becoming the male pronucleus. The egg nucleus after the last maturation division is called the female pronucleus. Each of these pronuclei, as a result of previous reduction, has the haploid or halved number of chromosomes and by the union of pronuclei to form the zygote nucleus the normal diploid number characteristic of the species is restored. Thus reduction maintains the specific number of chromosomes from generation to generation. A centrosome, the function of which is to initiate the process of cell-division, is also introduced by the spermatozoön, usually in the middle piece, replacing the egg centrosome which disintegrates after the last maturation division. The zygote, as the fertilized ovum is called, is now a complete cell, really a new individual in the stage of a unicellular embryo, with its chromatin, the vehicle of heredity, derived equally from the two parents.

Cleavage. Development of the zygote may be defined briefly as a progressive differentiation accompanied by cell-division and sooner or later by growth, but it must not be assumed that differentiation is determined by the cell division, for experimental embryology indicates

rather that the converse is the case. The term cleavage or segmentation is applied to the mitotic divisions by which the zygote is divided into numerous cells or blastomeres. When this process involves the entire zygote, it is described as total or holoblastic. In some cases the cells may for some time be equal in size, but where there is a unipolar aggregation of yoke, cleavage is mechanically retarded at the vegetal pole, the result being unequal cleavage, well shown in the egg of the frog, while if the yolk be very abundant cleavage may be partial or meroblastic, limited to a small disc of yolk-free protoplasm at the so-called animal pole, as in the hen's egg. In such cases this small disc, the blastodisc or blastoderm, gives rise to the entire embryo which gradually encloses, digests and absorbs the inert mass of yolk. In centrolecithal eggs of arthropods the cleavage is superficial over the entire egg. As a result of cleavage the egg in most cases soon attains the form known as the blastula, which in its most typical condition is a hollow sphere of cells containing a central segmentation cavity or blastocol. Where yolk is very abundant the blastula is greatly altered and in some forms there is no true segmentation cavity and strictly speaking no blastula. See Fig. 2.

Gastrula and Primary Germ Layers.-The single-layered blastula becomes transformed into a gastrula, a two-layered sac-like stage, in which there is an outer cell-layer calied ectoderm (or ectoblast) and an inner layer, the endoderm (or endoblast). This two layered stage is variously formed; in some cases, as in certain cœlenterates, cells wander inward from one pole of the blastula forming a solid inner mass which later becomes hollowed out, but a far commoner method of gastrulation is that known as the embolic type, in which a part of the gastrula wall, generally the part richest in yolk, becomes turned in or invaginated as a result of unequal growth to form a cup-like endoderm. The new cavity thus formed in the endoderm is the archenteron or primitive gut cavity; the mouth of the sac is the blastopore, which in various animals may form the mouth or the anus or neither. This simple sac-like gastrula is found only in eggs which have very little yolk, thus among vertebrates it is met with in typical form only in amphioxus, though readily recognizable in lamprey, amphibian and some other forms, while in most vertebrates the abundant yolk masks the sac-like character of this stage. Frequently in eggs with abundant yolk invagination of endoderm is mechanically impossible and in such cases gastrulation may be effected by an overgrowth of the ectodermal layer which surrounds the large yolk-filled portion of the egg. Such overgrowth is termed epiboly in contradistinction to emboly, or inturning of endoderm. However formed, the gastrula has considerable differentiation and foreshadows the orientation of the future body and some of the great organ systems. Its ectoderm is the source of the epidermis and the nervous system. The endoderm forms the lining of the gut and later gives rise to outgrowths which become the chief digestive glands. These two layers are called the primary germ layers and are of well-nigh universal occurrence. In those vertebrates which have very abundant yolk and consequent partial cleavage, as well as in mammals which seem to

retain the developmental mode of forms with large eggs, the two-layered stage is so modified as to be scarcely recognizable as a gastrula and in such cases the blastopore becomes compressed and drawn out into a longitudinal primitive streak which is almost the earliest evidence of the body axis.

Mesoderm. In all animals above the cœlenterates a third germ layer called the mesoderm (or mesoblast) develops between the two primary layers and gives rise to the connective tissue, muscles, blood system and gonads. This layer arises in very diverse ways. In many worms it is segregated very early in cleavage as special mesoblast cells. Usually it appears much later as a differentiation from the endoderm or in rare cases even from the ectoderm. In its origin from the endoderm it either delaminates as a sheet of cells from the outer surface of that layer, or arises as a series of hollow, sac-like outgrowths from the endoderm called enterocols or gut-pouches. When formed by the latter method the mesoderm from the beginning contains cavities which were originally parts of the primitive gut cavity. In cases where it splits off as solid masses similar cavities appear within it later. Such cavities in the mesoderm become the cœlome or true body cavity. In animals in which the body is segmented or metameric, such as the annelid worms, arthropods and vertebrates, the first evidence of segmentation appears in the mesoblast. In certain embryos a rather ill-defined tissue appears composed of loose cells and called mesenchyme. It may be produced very early, before the true mesoderm, or it may be proliferated from that layer. In general it gives rise to connective tissues.

Germ-Layer Theory.- All metazoa, excepting sponges and cœlenterates, exhibit three germ layers, a fact to which great significance has been attached by many embryologists. The sponges are so aberrant in their development that it is impossible definitely to identify their two layers with ectoderm and endoderm; while the cœlenterates, as suggested by Haeckel, may be regarded as a primitive group which has not progressed morphologically beyond the gastrula stage of complexity. The question of the homology of the three germ layers in the other phyla is one which has evoked much discussion and has led to considerable difference of opinion. As comparative embryology became known, the well-nigh universal occurrence of three layers and the general similarity of their respective derivatives naturally led to the assumption of their homology, a generalization known as the "germ-layer theory," though, as stated above, the middle layer differs greatly in its mode of origin in different groups. In nearly all cases, however, the ectoderm gives rise to the epidermis, the lining of the mouth and anal region, the nervous system, and in some invertebrates, to the kidneys. The endoderm, with which from the beginning the nutritive yolk is especially identified, becomes the lining epithelium of most of the alimentary canal and the chief digestive glands and in vertebrates gives rise to the germ cells which later wander into the mesoderm. The mesoderm, the latest layer to appear, is the source of the connective tissues, including the internal supporting hard parts when such are present, the blood and blood vessels, the muscular sys

tem, the gonads with the germ cells in most cases, and usually the kidney system. The methods by which germ layers become differentiated into their derivative tissues and organs are so varied that limitation of space precludes their present discussion, but it may be stated that common accompaniments of histogenesis are thickening, folding and delamination (splitting) of layers and also localized proliferation of free cells. The assumption of homology of the germ layers in different groups was quite natural, but of late years evidence has accumulated which indicates that many of the developmental resemblances of different phyla are to be interpreted rather as similar but quite independent reactions to like environmental factors; or in a word, as homoplastic rather than truly homologous.

Nutrition of Embryo.- Throughout the entire course of development the mechanical effect of food-yolk is very marked, not only in its retardation or prevention of cleavage in certain parts of the egg but in its mechanical effect on the formation of the germ layers and its physiological relation to development of the nutritive system. In general, though there are many exceptions, large eggs rich in yolk develop slowly and the resulting embryos hatch in an advanced state, often with essentially the adult form, while small eggs poor in yolk must early develop some means of securing food and usually in such cases the embryonic period is very brief, the embryo hatching in the form of a larva, often totally different from the adult. Such larvæ are especially common among marine invertebrates, in which usually they have the form of minute free-swimming organisms, often with no resemblance to the adult either in form or habit. Examples are the trochophores of annelids and molluscs, the nauplius of the crustacean, bipinnaria of the starfish, etc. In some cases the larva represents only a small portion of the future adult animal, occasionally only a portion of the head precociously equipped with an alimentary system and means of locomotion. These larvæ feed on various micro-organisms and eventually become made over into the adult form by a more or less complete metamorphosis.

Extra-embryonic Membranes.- In some animals extra-embryonic membranes are produced which subserve a temporary function in the protection or nutrition of the embryo and which are lost at hatching or at birth. In the higher vertebrates such structures include the chorion, a membrane forming the outer wall of the entire embryonic vesicle; the amnion, a closed water-sac lined with ectoderm and completely enclosing the embryo, and the allantois, an extension outside the body of the urinary bladder which in reptiles and birds and also in the primitive egg-laying mammals known as monotremes spreads its vascular wall inside the chorion close to the porous egg shell and serves physiologically as an embryonic respiratory organ. In the marsupial mammals, such as the kangaroo and opossum, the young are nourished during the very brief period of gestation by "uterine milk," a secretion of uterine glands which the embryo absorbs by means of its vascular membranes, chiefly the yolk sac. Uterine milk is also an important source of nutriment to the embryo even in some placental mammals, where it contains leucocytes and the detritus of