32 



COMPARATIVE ANATOMY 



CHAP. 



tHen becomes completely constricted off from the egg (Fig. 29) as the 

 first polar body. In a similar manner a second polar body is formed. 

 The formation of a polar body thus appears like a process of gemma- 

 tion, or a sort of cell division, in which one cell the daughter cell 

 the polar body, is very much smaller than the other the egg. 



ir 



VI 



FIG. 29. Formation of the polar bodies in Asterias glacialis, after O. Hertwig. sp, Nuclear 

 spindle ; rk\ first, rfc 2 , second polar body ; efc, female pronucleus. 



The half of the nuclear spindle which remains in the egg after the 

 formation of the second polar body changes into an egg nucleus very 

 different from the original germinal vesicle, especially in its size, being 

 much smaller. This germ is known as the female pronucleus. 



In many divisions e.g. many insects, there are species in which, alternating with 

 the sexual generations, which multiply by means of fertilised eggs, there occur other 

 generations, which reproduce by means of parthenogenetic eggs, i.e. by means of 

 such eggs as develop without fertilisation. In these eggs, according to some 

 observers, only one polar body is formed. Many hypotheses have been brought 

 forward as to the meaning jof all these various phenomena of maturation. I can 

 here only refer to the works of Biitschli, Balfour, Minot, Sabatier, van Beneden, 

 Weismann. 



Processes similar to those of the expulsion of the polar bodies from 

 the egg have been observed in the formation and ripening of the 

 spermatozoa. The nucleus of the ripe spermatozoon is called the male 

 pronueleus. 



When the polar bodies have been expelled the egg is capable of 

 fertilisation. 



Fertilisation. 



This process takes the following course ; out of numerous sperma- 

 tozoa pressing towards the egg there is only one, normally, which 

 fertilises it. This is the one which first touches the egg at a definite 

 point as it appears, viz. at the animal pole, in eggs differentiated into 



