Metasoa Rana. 



329 



*J>, 



FIG. 185. TYPICAL OVUM. (Quain.) 

 vi V s 



\ 



mentation then commences, the entire ovum undergoing 



division. Two cells, therefore, result, one half of each, 



however, owing to the distribution of the yolk, being more 



protoplasmic than the other. Occasionally (as in the bird) 



the relative abundance of yolk and comparative absence of 



protoplasm in that section of the ovum prevent the segmen- 



tation being complete, that is to say, only the protoplasmic 



part of the ovum undergoes segmentation, while the yolk 



remains passive and acts as a store of food-matter for the 



embryo which will de- 



velop from the more 



protoplasmic section. 



When segmentation is 



complete, as in the frog, 



it is spoken of as holo- 



blastic ; when it is in- 



complete, as in the fowl, 



it is spoken of as mero- 



blastic. The yolk por- 



tion of the frog's ovum 



divides much more 



slowly than the pro- 



toplasmic, so that as a 



final result of the divi- 



cinn i micscs nf rplk ic 

 SlOn a maSS C 



formed, small and very 

 numerous at the protoplasmic, large and fewer at the yolk 

 end. This will be best understood by reference to fig. 187. 

 The next change which takes place consists in the gradual 

 covering of the large slowly dividing cells by the rapidly 

 developing protoplasmic cells, until in the end the former 

 become entirely hidden save at one spot, the blastopore. 

 Although no actual invagination takes place as in Ampkioxus^ 

 yet it is not difficult to see the homologies of the parts of 

 the embryo, or the similarity of the phenomena in the two 

 cases. The ^mail-celled outer part is obviously epiblast 



cell-wall ; w, protoplasm with fat-grermles 

 5^ nucleus ;**, nucleolus. 



