38 



PRINCIPLES OF EMBRYOLOGY 



oocyte, so that the nurse-cells have almost withered away by the time 

 the oocyte is fully grown. 



The processes of oogenesis often include the production of special pro- 

 tective membranes to clothe the egg-cell, though a few types of marine 



Figure 2.5 

 Figures a, h, c show various types of 'egg-strings' from the ovaries of insects. 

 In a the eggs are in simple foUicles (e.g. Orthoptera); in h the foUicles are 

 accompanied by nests of larger 'nurse cells' (e.g. Coleoptera, Drosophila); in 

 c there is a larger group of nutritive cells at the end of the string, with chan- 

 nels leading to each egg (e.g. Hemiptera). (After Korschelt and Heider.) 

 Figure d shows the discharge of material from the nucleolus of the germinal 

 vesicle mto the oocyte cytoplasm in Limnea. (From Bretschneider and Raven 



1951.) 



eggs are shed completely naked. Egg membranes are of three kinds; 

 those which are strictly part of the ovum itself, being secreted by its outer 

 surface; those which are formed by folhcle cells; and those which may 

 be laid down by the oviduct during the passage of the egg away from the 

 ovary. The first kind are usually known as the vitelhne membrane, the 

 second as the chorion, while the third may have a variety of names such 



