DR. MARTIN BARRY'S RESEARCHES IN EMBRYOLOGY. 311 



near the centre of the ovisac. The germinal vesicle is here surrounded by the pecu- 

 liar granules of the ovisac, either immediately-}- (Plate V. fig. 10. c), or having minute 

 oil-like globules interposed J (fig. 14. and 15. (=*)). The latter, with a pellucid fluid, 

 indicate the incipient formation of the yelk around the germinal vesicle §. The oil- 

 like globules accumulate (fig. 15. (*)), and minute opake granules gradually present 

 themselves among them. When the formation of the yelk has thus proceeded to a 

 certain stage, two membranes are seen surrounding it ; one of which is the proper 

 membrane of the yelk, memhrana vitelli (49.), and the other, more external, is the 

 true c/ior/o/? II (52.) (fig. 17. e. and /".). The ovum is seen with great distinctness 

 through the transparent membrane of the ovisac, and it is thus possible to follow its 

 several stages of formation. Subsequently, a covering or tunic, consisting of a kind 

 of dense cellular tissue susceptible of becoming highly vascular, and closely connected 

 with the surrounding stroma, is gradually formed upon the outer surface of the ovisac, 

 which previously in a high degree transparent, now becomes translucent only. 



The Folliculus Graqfianus, or Graafian f^esicle. 



25. From the union of the ovisac with the covering or tunic now mentioned, I ap- 

 prehend it is that there results what has been called a Graafian vesicle ; and it ap- 

 pears to me that the " Couche interne de la capsule de la v^sicule de De Graaf" of 

 Baer (Plate VII. fig. 62. h.) is in reality the previously independent ovisac, while the 

 "couche e^itei^ne"^ of this author (Plate VII. fig. 62. i.) is the covering or tunic of the 

 ovisac above mentioned. 



26. Professor Baer has expressed his belief that the formation of the " ovulum" 

 (ovum) precedes that of the Graafian vesicle ; but he adds, " Je doute qu'il soit jamais 

 possible a I'homme de s'en convaincre par Tobservation-f"}-." I trust that the facts 

 mentioned in the preceding pages, and illustrated in Plate V., will suflSce to show how 

 far this eminent naturalist was right in his conjecture as to the priority in formation 

 of the ovum ; and that they will also serve to demonstrate the possibility (questioned 

 by Baer) of determining this point bi/ observation. 



t See the third column in the Table, page 304. J See the fourth column in the Table, page 304. 



§ The germinal vesicle is thus in or near the centre of the yelk when the latter is first formed. This vesicle 

 subsequently passes to the surface of the yelk, 



T. W. Jones (London and Edinburgh Philosophical Magazine, No. 39, Sept. 1835, p. 209.) mentions the 

 germinal vesicle as " having on one side a small elevation, which, projecting among the grains composing the 

 granular sac, fixes the vesicle in its place." I have never observed this " small elevation," nor do I find any 

 mention to have been made of it by either Coste or Valentin, who had previously described the germinal 

 vesicle in Mammalia, or by those who have since written on it. 



II See the fifth column in the Table, page 304. 



^ Von Baer says of this " couche externe," " Elle re9oit des vaisseaux, les contient dans son interieur, et 

 leurs extr^mites vont dans la couche suivante" (" couche interne"). Lettre, &c., p. 17. 



ft Lettre, &c., pp. 20, 21. 



