ITS DEVELOPMENT. 



35 



tured : but in ova of the human subject and some other animals the yolk is 

 much more consistent, and sometimes escapes as a solid globular mass 

 when the zona pellucida is torn. It is, according to Bischoff, solely 

 owing to this firm consistence of the yolk that it, in many cases, preserves 

 its form when a watery fluid passes by imbibition through the zona pellu- 

 cida, and that an interval is then apparent between the yolk and that 

 membrane. 



Owing to the tough consistence of the yolk in the human ovum, Bischoff 

 has not succeeded in isolating its germinal vesicle ; but he has satisfied 

 himself that it lies near the periphery of the yolk, though not imbedded in 

 a discus proligerus, as it is in the bird's egg.* - 



The germinal spot which lies at that part of the periphery of the germi- 

 nal vesicle' which is nearest to the periphery of the yolk, presents in the 

 mammiferous ovum no appearance of a vesicle or aggregation of cells, but 

 merely that of a finely granulated substance, of a yellowish colour, strongly 

 refracting the rays of light, f 



The subjoined table gives the measurements of the mammiferous ovum 

 and its different parts. 



Development of the ovum in the ovary. The process by which the ovum 

 is formed and brought to its state of maturity is scarcely noticed by Profes- 

 sor Miiller. The enquiries of Dr. Barry are not mentioned by him, and 

 those of Valentin only alluded to in a single line. It will be necessary, 

 therefore, to premise here some account of their observations before detail- 

 ing the results of the more recent researches of Bischoff. 



The questions which it seems most important to decide by the aid of 

 the facts revealed by these anatomists are the following : 



1. Is the Graafian follicle the immediate formative organ of the 

 ovum ' 



2. In what order are the different parts of the ovum formed ? 



3. What changes do they undergo in the progress of the ovum towards 

 maturity \ 



* Op. cit. p. 15. t Ibid. p. 15 and p. 556. 



D2 



