50 Dr Martin Barry's Researches in Embryology. 



thought, that the shape characteristic of the Bird's " egg" is 

 first intimated there — in the ovary. And if so, the shape in 

 question is after all not peculiar to the " egg" of the Bird ; for 

 it happens that the ovarian ovisacs to figures of which I am 

 now referring, were seen in one of the Mammalia.* 



From the observations of Von Baer and R. Wagner, in 

 invertebrated animals, and my own in two classes of the 

 Vertebrata, I concluded, in 1838, that the germinal vesicle 

 and its contents constitute, throughout the animal kingdom, 

 the most primitive portion of the ovum. "I 1 Subsequent re- 

 search in the Bird enabled me to record this as an established 

 fact. J And as the positions to be assigned to the several 

 parts of the ovum, in the language of " cells" have not yet 

 been satisfactorily determined, I will here, in that language, 

 state my own recorded observations. 



There first exists a pellucid particle, which becomes an 

 elliptical " cytoblast.' ' Out of the nucleolus of this " cyto- 

 blast'' there arise the germinal vesicle and its contents ; and 

 then the outer part of the " cytoblast" forms the membrane 

 of a cell, — my ovisac. To this cell the germinal vesicle is 

 related as the hollow nucleus to a ganglion globule. Out of 

 the granular contents of the cell now mentioned is formed, 

 first, a portion of the yelk around the germinal vesicle, and 

 then, around the incipient yelk, the vitellary membrane — 

 the "zona pellucida" of Mammalia — which arises in a semi- 

 fluid form. 



The occasional presence of two or more ova in a single 

 ovisac, is to be explained as follows. It sometimes happens 

 that before the formation of the membrane of the ovisac, the 

 nucleolus of the " cytoblast " has divided into two or more 

 parts, each of which becomes a germinal vesicle : and then 

 the membrane of the ovisac, subsequently formed, is made to 

 include the whole of these, — and we have in one ovisac two 



* The Dog. Phil. Trans., 1838, Part ii., Plate 8, fig. 74, h. 

 t " Researches in Embryology," First Series. Phil. Trans., 1838, Part ii 

 § 93. 



| Phil. Traas., 1841 ; Part ii. ; Plate 25, figs. 165 to 173. 



