in the Foetus of Vertehrated Animals. 303 



ovarium, but they were quite at a loss to conceive what part of 

 this vesicle the ova formed, as no direct observation had ever 

 been made of their presence there. We are indebted to Pro- 

 fessor Baer* for a solution of this difficulty, in the discovery 

 which he recently made of the ovulum in the interior of the 

 Graafian vesicle. Baer made this discovery first in the dog. 

 The ovulum has since been found in the human species, and in 

 all other mammiferous animals in which it has been sought for, 

 by Baer and by my friend Dr Sharpey, through whose kind- 

 ness I have had an opportunity of seeing it in the cow, sheep, 

 pig and rabbit. 



In the dog the ovulum, while in the Graafian vesicle, is so mi- 

 nute, that its structure cannot be easily examined. It appears 

 to be only about the 200th part of an inch in diameter. As 

 soon as it enters the oviduct it begins to enlarge, and it in- 

 creases in size as it passes along that tube towards the ute- 

 rus. In the uterus the growth of the ovum is proportionally 

 more rapid than before ; its structure may be examined with 

 ease when it is about half a line in diameter. At this time the 

 interior of the ovum is seen to be filled with a granular matter, 

 frequently of a yellowish colour, which is probably the substance 

 of the yolk : this matter is inclosed within a double covering, 

 the internal layer of which appears to correspond to the proper 

 membrane of the yolk, the external to the proper membrane of 

 the ovum, or chorion. According to Baer, Prevost, and Dumas, 

 the cicatricula or germinal membrane is visible at this time on 

 the surface of the yolk (Figs. I. and II.), and bears some re- 

 semblance to the corresponding part in the egg of the bird ; 

 being a round opaque granular disk, with a dark spot in its 

 centre. I have seen this spot very evident in the ova of the 

 rabbit on the 6th day after impregnation, (Fig. III). 



As development proceeds, the germinal membrane increases 

 in size ; its central part becomes thin and transparent ; its peri- 

 pheral part expands laterally, so as to form a cover for the yolk, 

 the yolk bag, intestinal sac, or what in mammalia has been called 

 the Umbilical Vesicle. 



According to Baer, and to Prevost and Dumas-f, the two last 



• Epistola de Ovi Mammal, et Horn. Genesi, Lipsise, 1827; also in the 

 Repertoire Gener. d'Anat. ct de Physiol, torn. vii. 

 t Annales des Scien. Natur., torn. iii. 



