Development of the zygote may be defined briefly as a progressive differentia tion 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 mi totic 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, cleav age 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 cen trolecithal 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 blastoccel. 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 single-layered blastula becomes transformed into a gastrula, a two-layered sac-like stage, in which there is an outer cell-layer called ecto derm (or ectoblart) and an inner layer, the endoderm 4or endoblast). This two layered stage is variously formed; in some cases, as in i certain coelenterates, 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 sonic 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 ectoderml layer which surrounds the large yolk-filled por tion of the egg. Such overgrowth is termed epiboly in contradistinction to emboly, or in turning 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 ec toderm is the source of the epidermis and the nervous system. The endoderm forms the lin ing of the gut and later gives rise to out growths which become the chief digestive glands. These two layers are called the pri mary 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 modi fied as to be scarcely recognizable as a gas trula 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 coelenterates 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 ecto derm. 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 endo derm called enteroccels or gut-pouches. When formed by the latter method the mesoderm from the beginning contains cavities which were 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 ccelome or true body cavity. In animals in which the body is seg mented or metameric, such as the annelid worms, arthropods and vertebrates, the first evidence of segmentation appears in the meso blast. 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 may be prolif erated from that layer. In general it gives rise to connective tissues.
All metazoa, except ing sponges and ccelenterates, exhibit three germ layers, a fact to which great• significance has been attached by 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 ccelenterates, as suggested by Haeckel, may be regarded as a primitive group which has not progressed morphologically beyond the gas trula 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 opin ion. 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 as sumption 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 endo derm, with which from the beginning the nutri tive 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 meso derm, 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 tern, the gonads with the germ cells in most cases, and usually the kidney system. The methods by which germ layers become differ entiated 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 prolif eration of free cells. The assumption of homol ogy 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 fac tors; or in a word, as homoplastic rather than truly homologous.