842 



PHYSIOLOGY. 



There are a few instances, however, in which the ovum alone is 

 capable of reproducing a new individual. This is observed only 

 among lower animals, and this method of reproduction is known as 

 parthenogenesis. In all more highly organized animals the ovum is 

 not capable of developing into a new individual, unless it comes in 

 contact and fuses with a part of another germinal cell called a sper- 

 matozoon. This method is called sexual reproduction-. In it the ovum 









Fig. 378. Graafian Follicle from Ovary of a New-born Child. 

 (After P. STRASSM AN. ) 



It consists of five portions: (1) An external membrane or zona pellucida. 



(2) An internal membrane or vitelline membrane which lies in close to the 

 yelk; between the two membranes is a slight space, the peri-vitelline space. 



(3) The yelk or vitellus, containing yelk grains or dentoplasm. (4) The nucleus, 

 germinal vesicle, vesicle of Purkinje. (5) The nucleolus, germinal spot, spot of 

 Wagner, consisting mainly of chromatin. 



presents the female element, the spermatozoon the male element of 

 reproduction. The process of meeting and ultimate fusion of the 

 two elements to form one, capable of forming a new individual, is 

 called fertilization. 



The fact that in producing a new individual a union of male 

 and female elements takes place has been known for centuries; but 

 in regard to the importance, or the predominating influence, of the 

 one or the other of these elements, the views have changed. During 



