152 



PRINCIPLES OF EMBRYOLOGY 



was tilted. The blastopore appears at that edge of the yolky mass which is 

 nearest to this position (Fig. 9-3^). 



Pasteels (see also Dalcq 1950/;) concludes that the properties of the grey 

 crescent and thus eventually of the blastopore are brought about by the 

 interaction of two factors. The first is a gradient in cytoplasmic constitu- 



RAN. 



P.VEG. 



RAN. 



RAN. 



Figure 9.4 



The relation between the main yolk-mass (dotted) and the anterior-poster- 

 ior axis of embryos developed following rotation of the frog's egg. The 

 arrows point to the cephalic region of the mesoderm ; a is the normal situa- 

 tion; b, c and d show inverted eggs in which there is little, considerable or 

 complete redistribution of the yolk to lower pole. Note that the cephalic 

 region of the mesoderm (i.e. the blastopore) always forms near the yolk- 

 mass. (From Pasteels 1951.) 



tion which normally runs from the animal pole (rich in cytoplasm and 

 cytoplasmic granules, and poor in yolk) to the vegetative pole (large yolk 

 platelets and little cytoplasm). This gradient determines the cephalo- 

 caudal polarity of the embryo which will develop. It interacts with a 

 cortical field which has a point of highest activity in the future grey 

 crescent region and falls off from that in all directions (Fig. 2.7, p. 42). 

 It has been necessary to dwell at some length on these early events since 



