44 



PRINCIPLES OF EMBRYOLOGY 



the position in which it lies in the ovary. This question has not had as 

 much attention in recent times as it deserves. It is known that in many 

 insects with elongated, banana-shaped eggs, the long axis of the egg 

 always lies along the long axis of the insect; in some invertebrates, the 

 position of the egg nucleus and of the accumulation of yolk has a definite 

 relation to the blood supply, and the same has been claimed to be true of 

 frogs eggs. But in most of the types of eggs in which the internal structure 

 is clearest (e.g. ascidians) we know very little about the geometry of their 

 formation within the ovary. 



Figure 2.8 



The 4-cell stage and the adult shell of dextral {1, 2) and sinistral (3, 4) Linmea 



peregra. (After Robertson 1953.) (s) Shows part of the tail o£Limnea sperm 



consisting of three large and one small strand wound, always in a dextral 



spiral, round a central core. (After Sehnan and Waddington 1952.) 



SUGGESTED READING 



Rothschild 195 i<j. 



