68 



EMBRYOLOGY. 



_ d* 



Fig. 38. Blastula of Amphioxus, after HATSCHEK. 

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

 with abundant yolk. 



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



of tho 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 blastula or blfis- 



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 tho 



blastulae at this stage aban- 

 don the egg-envelopes, and, 



since their cylindrical cells 



develop cilia at the surface, 



-.vim about with rotating 



motion in the water as cilia te 



spheres or blastospheres. 



In eggs with unequal seg- Fig. 37.-BU.tuia oi Tm< 



mentation the blastula is fh> ^^n^r^k! 



ordinarily formed of several 



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



tho same time it exhibits in dilfrrent regions different thickn- 



'(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, 



. cel! 



