DEVELOPMENT OF THE FROG 125 



merited cells, and the floor of much larger irregularly-poly- 

 gonal yolk-cells which occupy the whole of the lower 

 hemisphere of the embryo. The small cells pass, without any 

 sharp line of demarcation, into the yolk-cells. Such a hollow 

 vesicle is known as a blastula, and its cavity is known as the 

 segmentation cavity or blastocoele. Further changes, brought 

 about by repeated cell-division, result in the formation of 

 a three-layered embryo, and all the organs and tissues of the 

 tadpole, and, finally, of the adult, are derived from these three 

 layers. 



The actual formation of the three layers is somewhat ob- 

 scured in the frog because of the large preponderance of 

 inert food-yolk, and it will be better to defer a consideration 

 of the details of further development till a later stage. 



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