FEKTILISATION AND SEGMENTATION OF THE EGG. 101 



the protoplasm in which it is imbedded, acting in exactly the 

 same way as so many grains of sand or other foreign matter 

 would do, and actually checking the processes of development. 



The frog's egg is a telolecithal egg ; i.e. one in which the food- 

 yolk is not uniformly distributed throughout the yolk, being 

 more abundant in the lower or white hemisphere than in the 

 upper or black one. The passage from one pole to the other is a 



FIG. 46. 



FIG. 47. 



FIG. 48. 



FIGS. 46, 47, and 48. Segmentation of the Frog's Egg. x 20. 



Fig. 46. The egg just before the completion of the first cleft, by which it is divided 

 into two equal blastomeres : the egg is represented in vertical section. 



Fig. 47. A surface view of an egg at the completion of the third cleft : the egg is 

 now divided into eight blastomeres, an upper tier of four small ones, and a lower tier of 

 four much larger ones. 



Fig. 48. A vertical section of an egg at the same stage as Fig. 47. In the middle, 

 between the inner ends of the blastomeres, is the commencing segmentation cavity. If. 



gradual, not an abrupt one ; there is no line of demarcation be- 

 tween the two ; still, the black pole is much less encumbered 

 with food-yolk than the white or lower pole, and it is to this fact 

 that the relatively rapid development of the black pole is due. 

 Very shortly after the completion of the act of fertilisation 



