710 



ANATOMY OF VERTEBRATES. 



555 



of observations 1 by which the ' germinal vesicle,' the ( germina 

 spot,' the yolk, and yolk-membrane, were determined in the 

 minute opaque sphere ; and thus was the ovarian egg of the 

 Mammal finally made known. 



The ovisac and ovum appear later in the ovary than do the 

 seminiferous tube and spermatoon in the testis. The first-formed 



elements in the fetal ovary are 

 those called ' cells ' and ' cell- 

 nuclei ? : next appear roundish 

 groups of such primary cells, 

 rather more opaque than the rest 

 of the previously uniform mass, 

 fig. 555, A. A film soon conden- 

 ses round these purposive groups, 

 ib. b, upon the inner surface of 

 which forms an epithelial preci- 

 pitate from the fluid and granules 

 of the interspaces of the contained 

 primary cells : within the ' ovisac ' 

 thus formed a larger nucleate cell 

 becomes visible, which is the be- 

 ginning of the ovum. As the ovi- 

 sac expands the proportion of fluid 

 to the formified particles increases, 

 and the latter are attracted to the 

 contiguous surfaces, some to that 

 of the ovisac, which thus becomes 

 lined by a thicker layer of cells, 

 others to the ovum, accumulating 

 around it. With the enlarge- 

 ment of the ovisac, the ' stroma 

 ovarii ' condenses around its deli- 

 cate membrane, fig. 556, b, to form 

 the ' theca folliculi ' of Baer. 

 This vascular covering of the 

 ovisac, ib a, with the proper 

 wall, ib, b, constitutes the ( Graafian vesicle or follicle.' The 

 stratum of nucleate cells lining the ovisac is termed f membrana 



voce o vuli semper usus sum, quia vesieula Graafiana ipsa ovum refert, respecto 

 ovario, ex ovulo autem, fit ovum fetale.' . . . ' Ex quo conclude* : quo diutms in corpore 

 materno fetus foventur, eo magis jam primitus exculta videtur ovi vesieula innata, quae 

 in mcunmalibus eo pervenit ut omnrs ovi virtutes in sese recipiat et reliquce ovi pattes 

 parvi momcnti extranece quasi fiant? ccxlix". p. 33. [The italics are V. Baer's.] 

 1 Ccl". ccli". cclii". 



Formation of the ovisac, Dog. cclxi", 



