CHAPTER I. 



THE OVUM AND SPERMATOZOON. 



THE OVUM. 



THE complete developmental history of any being constitutes 

 a cycle. It is therefore permissible in treating of this history to 

 begin at any point. As a matter of convenience the ovum ap- 

 pears to be the most suitable point of departure. The question 

 as to the germinal layer from which it is ultimately derived is 

 dealt with in a subsequent part of the work ; the present chapter 

 deals with its origin and growth. 



General History of tJie Ovum. 



Every young ovum (fig. i) has the cha- 

 racter of a simple cell. It is formed of a 

 mass of naked protoplasm (a), containing 

 in its interior a nucleus [b], within which 

 there is a nucleolus (c). The nucleus and 

 nucleolus are usually known as the ger- 

 minal vesicle and germinal spot. 



The ovum so constituted is developed 

 either (i) from one cell out of an aggrega- 

 tion or layer of cells all of which have the 

 capacity of becoming ova ; or (2) from one 

 out of a number of cells segmented off 

 from a polynuclear mass of protoplasm, not divided into sepa- 

 rate cells. In both cases the cells which have the capacity of 

 becoming ova may be spoken of as germinal cells, and in the 

 case where the ova are ultimately developed from a poly- 



B. II. 2 



FIG. i. DIAGRAM OF 

 THE OVUM. (From Ge- 

 genbaur.) 



a. Granular proto- 

 plasm. l>. Nucleus (ger- 

 minal vesicle), c. Nu- 

 cleolus (germinal spot). 



