68 



EMBRYOLOGY. 



Tig, 36. — BUstula of AmphioxU3, after Hatsohek. 

 A, Segmentation-cavity ; az, animal cells ; dz, cells 

 with abundant yolk. 



cavity increases more and more in size, so that the surface 

 of the whole germ is augmented, and the cells which were at 



first central come to the 

 surface. 



Different names have been 

 given to the solid and to the 

 hollow mass of cells. A 

 morula or mulberry-sphere 

 is spoken of as long as the 

 segmentation-cavity is either 

 wanting or only slightly de- 

 veloped. But when a larger 

 cavity has been formed, as 

 is almost always the case 

 toward the end of the 

 cleavage-process, the germ 

 is called a blastvla or hlas- 

 tosphere (Keimblase). The latter in turn exhibits a four-fold 

 variation of form, according to the abundance of yolk in the 

 original egg and the method of the antecedent segmentation. 



In the simplest case (fig. 36) the wall of the blastula is only one 

 layer thick ; the cells are of uniform size and' cylindrical, and are 

 closely united to one another 

 to form an epithelium (many 

 of the lower animals, Am- 

 phioxus). In the case of 

 lower, aquatic animals the 

 blastulse at this stage aban- 

 don the egg-envelopes, and, 

 since their cylindrical cells 

 develop cilia at the surface, 

 swim about with rotating 

 motion in the water as ciliate 

 spheres or blastospheres. 



In eggs with unequal seg- 

 mentation the blastula is 

 ordinarily formed of several 

 layers of cells, as in the case of the Frog and Triton, and at 

 the same time it exhibits in different regions different thicknesses 

 (fig. 37). At the animal pole the wall is thin ; at the vegetative 

 pole, on the contrary, it is so much thickened that an elevation,. 



Fig. 37. — Blastula of Triton tseniatus. 

 th, Segmentation-cavity ; rz, marginal zone ; tfo, cells 

 witli abundant yolk. 



