THE vertebrates: the amphibia and birds 



151 



presumably it was preceded by the formation of something corresponding 

 to a grey crescent, although it is not always possible to recognise this 

 clearly. The position of the blastopore and of the putative grey crescent 



an. 



Figure 9.3 



A. The effects of partial rotation of the uncleaved axolotl egg round an axis 

 perpendicular to the dorso- ventral plane. The small arrow points to the 

 grey crescent and the dots indicate the heavy vegetative ooplasm. Note 

 that the blastopore always appears at the margin of this, in whatever 



position is nearest to the original grey crescent. (From Pasteels 195 1.) 



B. The movement of marks on the cortex towards the animal pole (P-a.) 

 during the formation of the grey crescent. (From Pasteels 1951, after Ancel 



and Vintenberger 1948.) 



is now, however, not always determined by the direction in which the 

 yolk slid down to the bottom of the egg. It is influenced rather by the 

 position of the original grey crescent which had formed before the egg 



