722 



GENERATION AND DEVELOPMENT. 



Such are the parts of which the Graafian follicle and 

 its contents, including the ovum, are composed. The 

 diagram (fig. 203) represents them in their relative posi- 

 tions when mature. With regard to the mode and order 

 of development of these parts there is considerable un- 

 certainty; but it seems most likely that the ovum is 

 formed before the Graafian vesicle or ovisac. 



With regard to the parts of the ovum first formed, it 

 appears certain that the formation of the germinal vesicle 

 precedes that of the yelk and zona pellucida, or vitelline 

 membrane. Whether the germinal spot is formed first, 

 and the germinal vesicle afterwards developed around it, 

 cannot be decided in the case of vertebrate animals ; but 

 the observations of Kolliker and Bagge on the develop- 

 ment of the ova of intestinal worms show that in these 

 animals, the first step in the process is the production of 

 round bodies resembling the germinal spots of ova, the 



* Fig. 203. Diagram of a Graafian vesicle, containing an ovum. I. 

 Stroma or tissue of the ovary. 2 and 3. External and internal tunics 

 of the Graafian vesicle. 4. Cavity of the vesicle. 5. Thick tunic of the 

 ovum or yelk sac. 6. The yelk. 7. The germinal vesicle. 8. The 

 germinal spot. 



