SYMMETRY OF THE EGG, ZYGOTE, AND FUTURE EMBRYO 85 



its entrance into the egg, the more extensive will be the opposite gray 

 crescent. And the nearer it is to the animal pole the less apparent will 

 the gray crescent be. This gray crescent appears in about one hour 

 after insemination. It may develop earlier and be more extensive in 

 aged eggs. While it will move somewhat, by the process of epiboly 

 (e.g., downgrowth of surface materials), it will ultimately be found 

 in the region of the blastopore and the anus. This is the region of origin 

 of most of the mesoderm. It can be seen readily with the naked eye or 

 under low magnification, but it is difficult to detect in the sectioned 

 egg because the pigmented layer is relatively very thin. These de- 

 scriptions of egg polarity and rotatory symmetry, and of the secondarily 

 imposed bilateral symmetry, are of fundamental importance in under- 

 standing the development of the normal embryo. However, factors of 

 unequal pressure on the egg in any clutch of eggs may so alter the 

 position of the nucleus and the distribution of yolk and or cytoplasm 

 that the bilateral symmetry of the fertilized egg and the early cleavage 

 planes may not be so causally related. Even so, normal embryos will 

 develop. 



