ITS DEVELOPMENT. 37 



of the membrana vitelli, and zona pellucida (now regarded as one mem- 

 brane). The ovisac, or Graafian vesicle, subsequently acquires an external 

 vascular tunic composed of dense cellular tissue. 



M. Bischoff* agrees with Dr. Barry that the development of the Graa- 

 fian follicles and ova continues uninterruptedly from birth to the end of 

 the fruitful period of woman's life. In some animals, as the cow and sow, 

 it commences in the embryo, even at an early period of uterine existence, 

 but in the dog and rabbit, according to his observation, not till after 

 birth. 



M. Bischoff describes the process of formation of the Graafian follicles 

 and ova to be as follows : At first nothing can be distinguished in the 

 substance of the ovary but primary cells and nuclei of cells. Then 

 round groups of similar cells are seen scattered in large numbers through 

 the stroma. The peripheral cells of each of these groups subsequently 

 coalesce so as to form a homogeneous -transparent vesicular membrane, 

 while the portion of the mass within becomes fluid. Thus is formed the 

 Graafian follicle. On the inner wall of this follicle or vesicle new cells are 

 formed in the manner of an epithelial layer, while the cavity is found to 

 contain a transparent fluid with nuclei of cells and granules, exactly resem- 

 bling yolk granules, suspended in it. The next stage is marked by the 

 appearance of a second smaller transparent vesicle within the Graafian 

 vesicle. This second vesicle, which is the germinal vesicle, has a nucleus, 

 the germinal spot. Granules, similar to yolk granules, soon accumulate 

 around the germinal vesicle ; but the further steps in the development of 

 the ovum could not be traced. All its parts were completely formed 

 when M. Bischoff next observed it. 



From the preceding account of the observations of Valentin, Barry, 

 and Bischoff, it will be seen that the first and the last of these en- 

 quirers agree as to the fact of the ovum being developed within the 

 Graafian vesicle as its immediate formative organ, although they differ as 

 to the process by which the Graafian vesicle itself is formed. Bischoff 

 regards the statement of Dr. Ba.rry that'the germinal vesicle of the ovum 

 exists before the Graafian follicle as altogether an error. 



With regard to the second question proposed at page 35, namely, in 

 what order are the different parts of the ovum formed ? it appears a matter 

 of certainty that the formation of the germinal vesicle precedes that of 

 the yolk, and the yolk membrane. The observation of Dr. Barry as to 

 this point is entirely confirmed by Bischoff. 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 some recent observa- 

 tions of Kolliker \ and Bagge J on the development of the ova of intes- 

 tinal worms show that in these animals the first step in the process is the 



* Entwickelungs-geschichte, p. 365. t Miiller's Archiv. 1843, p. 72. 



$ Diss. de Evolut. Strongyli auricular, et Ascaridis acuminat. Erlangse, 1841. 



