128 COMPARATIVE ANATOMY 



blastomere is divided into two longitudinally, and the division 

 lines are continued downwards into the yolk blastomeres. 

 But these, being so largely composed of inert deutoplasm, 

 divide slowly, and their division is completed some time 

 after it has been effected in the upper blastomeres. The 

 next divisions are parallel to the equator, . and affect all 

 the sixteen blastomeres, dividing each of them into two, as 

 is shown in fig. 26, C. The embryo now consists of thirty- 

 two cells, and the succeeding divisions become irregular, and 

 are no longer synchronous. The upper pigmented cells divide 

 much faster than the lower yolk-cells, and the final result of 

 the first phase of development (usually called the segmentation 

 phase) is a vesicle containing a rather flattened and excentric 

 cavity, whose roof is composed of two layers of small pig- 

 mented cells, and the floor of much larger irregularly- 

 polygonal 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, follow- 

 ing upon 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. 



