THE BEGINNINGS OF LIFE. 



20 



Referring now, for a time, to the other mode of 

 formation of the ovum, we may state that the question, 

 concerning which there is the most uncertainty (and 

 at the same time one to which a considerable 

 interest attaches) is c whether the ovisac is to be re- 

 garded as the vesicle of evolution of the ovum, or 



st 



FIG. 13. 

 Diagrammatic representation of section of two Graafian Follicles or 



Ovisacs in different stages of advancement in the ovary of a mam- 



mifer ; enlarged about ten diameters. (^Coste.) 

 />. Peritoneal covering of the ovary. 

 st. Ovarian stroma. 

 ov. The two layers of the ovisac. 

 mg. Membrana granulosa, near which is the discus granulosus, with the 



ovum embedded. 



whether the ovum, or parts of it at least, are previously 

 formed, and the ovisac is afterwards superadded l ?' 



1 ' Cyclopaed. of Anat. and Phys.' vol. v. p. 554. In its later stages 

 the ovum of all the higher animals is found to be contained within 

 a most distinct ovisac or Graafian vesicle that is, within a com. 

 paratively large receptacle filled with a granular fluid, in which the 



