150 GENERAL EMBRYOLOGY 



line membrane depends upon the time relation between mem- 

 brane formation and maturation. The polar bodies may 

 form before the membrane, in which case they usually are lost 

 from the egg soon after their formation. More frequently 

 they form after, and therefore within, the membrane and so 

 can be seen for some time after development begins, when 

 they form useful points for orientation, for in nearly all cases 

 they mark the animal pole of the egg. The only exceptions to 

 this location are among the Insects and Copepods, in which 

 their position is variable. 



Morphologically there is also complete correspondence 

 between the ovum and the three polar bodies formed from the 

 primary oocyte, and the four spermatids and spermatozoa 

 formed from the primary spermatocyte (Platner, O.Hertwig). 

 But again there is physiological divergence, in that all of the 

 derivatives of the spermatocyte are capable of functioning as 

 germ cells, while only one of the oocyte derivatives may do so. 

 This physiological divergence is an expression, in another form, 

 of the physiological division of labor between the egg and the 

 sperm already referred to. 



The chief points of similarity and difference between ovum 

 and spermatozoon are summarized in the accompanying table. 



Little is known regarding the nature of the stimuli which 

 lead to the process of maturation, but it is clear that they are 

 quite varied in different eggs. In some of those cases where the 

 eggs are discharged freely into the water, contact with the 

 water seems to initiate the process. But maturation may be 

 begun previously to such a discharge. In some cases of this 

 kind, as well as in others where the eggs are not thus freed, the 

 rupture of the egg follicle seems to start the maturation process. 

 In many cases the entrance of the sperm into the ovum is the 

 effective stimulus to maturation, or to the completion of 

 maturation in many of those instances where it has been begun 

 previously and has been inhibited, either just before or just 

 after the formation of the first polar body. 



While we must postpone a part of our brief discussion of the 

 theoretical significance of maturation, for reasons stated 



