710 



ANATOMY OF VERTEBRATES. 



555 



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

 spot,' the yolk, and yolk-membrane, Avere 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. 55 5 , 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 



Formation of tlie ovis;ic, Dog. CCLXI". 



wall, ib. />, constitutes the e Graafian vesicle or follicle.' The 

 stratum of nucleate cells lining the ovisac is termed ' membrana 



vocc ovuli S9mper usus sum, qnia vesicula Graafiana ipsa ovum refert, respecto 

 ovario, ex ovulo autom, fit ovum fetale.' . . . ' Ex quo conclude: quo diutius in corpore 

 mater no fetus foventur, co mac/is jam -prim it us exculta videtur ovi vesicula innata, qure 

 in mammalil>i(s co perrenit ut onm<s ovi virtutcs in sese recipiat (t reliqu(g ovi paites 

 2>arvi momcnti extranet? quasi fiant? CCXLIX". p. 33. [The italics are V. Baer's.] 

 1 CCL". cci.i". cci.ii". ' 



