308 DR. MARTIN BARRY'S RESEARCHES IN EMBRYOLOGY. 



differs at different periods in the same animal. Yet notwithstanding these varieties 

 there is always found a general resemblance, so that when once examined these gra- 

 nules cannot fail to be recognised -f-. 



The Fluid contained in the Ovisac. 



11. This fluid is pellucid, often yellowish, partially coagulable, and is generally 

 considered to be albuminous. It contams a large quantity of the peculiar granules 

 just described, and a varying quantity of oil-like globules. This fluid having been 

 already described by others:}: I need not add more respecting it. 



On the Germinal Vesicle and its contents as the most primitive portion of the Ovum. 



12. It has appeared desirable to give a description of the ovisac the first place in 

 this memoir, but there is a portion of the ovum which exists before it. Each of the 

 many ovisacs already referred to, such as those in Plate V. fig. 4. h., and even the mi- 

 nutest of those in fig. 9. h., probably contained, besides the peculiar granules visible 

 (8.) in their interior, a concealed part, which, supposed to be the most important, also 

 appears to be the most primitive element of the ovum. 



13. PuRKiNjE, the first discoverer of the germinal vesicle in any animal (the Bird), 

 having observed that its relative size was greater, the more minute its containing 

 ovum, has expressed the opinion that it is the first part of the ovum formed §. Baer, 

 the discoverer of this vesicle in other oviparous Vertebrata, as well as in Moliusca, 

 Annelida, Crustacea, and Insects ||, believes that he observed its formation in animals 

 belonging to the two first-mentioned classes to precede the evolution of the ovum^. 

 R. Wagner has described and figured the posterior extremity of the oviduct in in- 

 sects {Acheta campestris)'^-^ as filled with little besides germinal vesicles, each of which 

 contains a spot ; and in a later publication:}: J he has shown similar objects in one of 

 the Libellulae {Agrion), and also in the freshwater Beetle {Dyticus marginalis). 

 These observations of Baer and Wagner I have been enabled to extend certainly to 

 two, and I think to three classes of the Vertebrata. 



14. Plate V. fig. 1., taken from the Rabbit, presents vesicles, c, containing fluid, 

 each of which has its own envelope, consisting of the peculiar granules, g, just de- 

 scribed (8.). These vesicles, of which a considerable number was seen in the same 

 field of view, were for the most part elliptical, some of them nearly globular, their 

 long diameter varying from the 130th to the 50th of a Paris line§§. The contained 



t Several writers have made mention of these granules, but an adequate description of them I do not find 

 to have been given. 



X Baer, Lettre, &c., pp. 18, 19. Valentin, Handbuch der Entwickelungsgeschichte, &c., S. 15, 16. Bern- 

 hardt, Symbolse ad Ovi Mammalium Historiam ante Prsegnalionem, pp. 10, 11 ; Vratislavise, 1834. 



§ Encyclopadisches Worterbuch, Band x. S. cxi. 



11 Burdach, Die Physiologic als Erfahrungswissenschaft, Band i. § 63. 1835. 



^ Lettre, &c., p. 21. ff Prodromus, &c., fig. xviii. a. XX Beitriigc, &c.. Tab. ii. figs. 1 to 6. 



§§ = TTrVr to -TTT of an English inch. 



