DEVELOPMENT OF RANA 



173 



(due to the retreat of pigment into the egg) at the point dia- 

 metrically opposite to that at which the sperm entered. The 

 egg can now be orientated with regard to the axes of the future 

 embryo. The animal pole will become the head, and the 

 vegetative pole the tail ; the grey crescent marks the future 



A 



B 



C D 



Fig. 73. — Egg of Rana temporaria (common frog) before and after fertilisa- 

 tion, showing the formation of the grey crescent. (From Jenkinson.) 



A and B seen from the side ; C and D seen from below ; A and C 

 before and B and D after fertilisation. The animal hemisphere is pig- 

 mented, the vegetative hemisphere is light in colour. 



dorsal side, and the opposite side (where the sperm entered) 

 will be ventral. 



Cleavage. — Cleavage in the frog's egg is total, but the size 

 of the various blastomeres is very unequal, owing to the large 

 quantity of yolk. The cells at the vegetative pole are much 

 larger (and fewer in number) than those at the animal pole. 

 The blastoccel is small, and situated nearer to the animal than 

 to the vegetative pole. The bias tula is now a hollow ball, 

 but the hollow is small and its walls are several layers thick. 



Gastrulation. — The cells of the animal hemisphere (which 



