THE REPRODUCTION OF ANIMALS 235 



planes, and this results in an 8-cell stage. At the fourth cleavage each 

 cell is divided into two by planes that pass through both poles of the 

 embryo, to produce a 16-cell stage. The fifth cleavage results in a 32-cell 

 stage; the sixth, in a 64-cell stage, etc., but later cleavages are not likely 

 to be so regular as the first four or five. 



The blastula. The rapidly multiplying cells of the embryo show a 

 marked tendency to round off their acute inner angles so that a cavity 

 is formed at the center of the spherical mass of cells. As cleavage follows 

 cleavage, this cavity increases in size until, by the end of the cleavage 

 period, when the embryo consists of several hundred cells, it has the 

 form of a hollow ball. At this stage the embryo is known as a blastula 

 and its central cavity, which is filled with fluid, as the blastocoele. 



Gastrulation. The completed blastula consists of a single layer of 

 cells surrounding the blastocoele; but almost as soon as it is formed, 

 it begins to change into a two-layered structure known as a gastrula. 

 This change is accomplished by a flattening and then an indenting (in- 

 vagination) of one pole of the blastula, until the blastocoele is obliterated 

 and the cells of the indented side of the blastula wall are in contact with 

 those of the opposite side. This indenting process, or invagination, is 

 accompanied by a rapid multiplication of cells, so that the double-walled 

 (diploblastic) gastrula has approximately the same outward shape and 

 size as the blastula from which it developed. 



The outside wall of the gastrula is known as the ectoderm; the inner 

 wall, as the entoderm; the latter lines a new cavity, the archenteron, which 

 opens to the outside of the embryo through an opening, the blastopore. 

 Modification of cleavage and gastrulation. In telolecithal eggs the 

 processes of cleavage and gastrulation, although clearly comparable to 

 those of the homolecithal egg, are modified by the large quantities of 

 inert yolk. In the frog's egg the early cleavage stages are not markedly 

 different from those described above, but the later cleavages are unequal 

 and result in a blastula with many small cells at the animal pole and 

 few and larger cells at the opposite pole. The blastula wall is much thicker 

 in the region of these large yolk-filled cells, and the blastocoele is thus 

 displaced toward the animal pole. In more strongly telolecithal eggs 

 cleavage is confined to the small cytoplasmic disk at the animal pole, and 

 the blastocoele forms a small shallow cavity beneath this disk. Gastrula- 

 tion is correspondingly modified, considerably in the case of the frog 

 embryo and greatly in embryos that develop from strongly telolecithal 

 eggs. Nevertheless, a true gastrula is always formed with a distinct 

 differentiation of outer ectoderm and inner entoderm and the formation 

 of an archenteron and blastopore. 



Mesoderm formation. In all but the two lowest phyla of the Metazoa 

 the completion of the diploblastic gastrula stage is immediately followed 



