GENERAL EMBRYOLOGY 1603 



to the mature ovum of the female. Each spermatid now undergoes 

 transformation into a spermatozoon, the change taking place within 

 a cell or column of Sertoli (sustentacular cell). Prior to its full 

 development a spermatozoon has passed through four stages — 

 namely, (i) a spermatogonium; (2) a primary spermatocjle ; (3) a 

 secondary spermatocele ; and (4) a spermatid. In all cases the cell- 

 divisions are of the nature of a somewhat modified mitosis or kar\'o- 

 kinesis, consisting in a reduction-division — ^that is to say, a reduction 

 in the number of chromoses at each cell-division. 



From one primary spermatocyte (mother-cell) there thus result 

 four grand-daughter cells of equal size, each of which is a spermatid. 

 These spermatids subsequently imdergo transformation, each into 

 an active spermatozoon, capable of fertilizing a mature o\ami. 



The Ovum. 



The ovum or oocyte, which is the female germ-cell, has all the 

 characters of a typical cell, being specially remarkable for the large 



Genninal Spot (Nndeolns) 



Germinal Vesicle (Nuclens) 



Gsrona Radiata 

 Zona Radiata 



Perivitelllne Space 



Vitellus or Yolk 



Perivitelline Membrane 



Fig. 660. — ^The Ovtm ied after Stricker). 



size of its nucleus and nucleolus. It is formed within a Graafian 

 follicle of the ovary, and it has a diameter of y^ inch. Its com- 

 ponent parts are as follows: 



1. Cell- wall. 3. Nucleus. 



2. Cell-body. 4. One or more nucleoli. 



The cell-wall is knowTi as the vitelline membrane. 



The vitelline membrane surrounds the vitellus, of which it is a 

 peripheral condensation. External to the vitelline membrane is 

 the zona pellucida, which is separated from the vitelline membrane 

 f by a narrow interval, called the perivitelline space. When examined 



