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 appears 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 the Ovum. 



Every young ovum (fig. 1) has the character 

 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 germinal vesicle and germinal 

 spot. 



The ovum so constituted is developed either 

 (1) from one cell out of an aggregation or layer 

 of cells all of which have the capacity of be- 

 coming ova ; or (2) from one out of a number 

 of cells segmented off from a nucleated mass 

 of protoplasm, not divided into separate 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 nucleated mass of protoplasm the 

 latter structure may be called a germogen. 



In some cases the whole of the germinal cells eventually become 

 ova, but as a rule only a small proportion of them have this fate, the 

 remainder undergoing various changes to be spoken of in the sequel. 

 Extended investigations have shewn that the distinction between 



FIG. 1. DIAGRAM OF 

 THE OVUM. (From Ge- 

 genbaur.) 



a. Granular proto- 

 plasm, b. Nucleus (ger- 

 minal vesicle), c. Nu- 

 cleolus (germinal spot). 



