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mulus ; and, 4thly, the ovum, which is contained in the cavity just 

 mentioned. The human ovum is so small as to be only just percep- 

 tible to the naked eye, being the 150th part of an inch in diameter. 

 It has a soft transparent envelope of considerable thickness, and 

 contains a substance composed of grains, adhering together by the 

 intervention of a delicate mucous tissue. At the inner surface of the 

 envelope, the author discovered a delicate transparent vesicle, about 

 the 900th part of an inch in diameter, and having on one side a small 

 elevation, which, projecting among the grains composing the walls of 

 the granular sac, fixes the vesicle in its place. The author considers 

 this vesicle as being analogous to that described by Professor Pur- 

 kinje in the cicatricula of the immature eggs of birds, and which ex- 

 ists also in the ova of other oviparous animals, and is termed by Baer 

 the germinal vesicle. 



The author has also examined the ova of the cow, sheep, sow, rab- 

 bit, rat, and mouse; and has found in all these animals a germinal 

 vesicle, differing in no essential particular from the human structure, 

 and in size bearing a proportion to that of the ovum as one to six. 



Although there is, at first sight, a considerable resemblance be- 

 tween the nucleus of the vesicle of De Graaf and the immature yelk 

 of the egg of a bird, the author thinks, contrary to the opinion of 

 Baer, that there is no real analogy between them ; because, in the 

 Graafian vesicle of the Mammalia there is no membrane surrounding 

 its nucleus similar to the vitellary membrane of the ovum in birds, 

 nor does this latter membrane appear first under the form of a gra- 

 nular membrane. The vesicle of Purkinje consists merely of a delicate 

 capsule containing a fluid; while in the minute ovum of Mammalia 

 there are found all the essential elements of the egg of birds and other 

 Ovipara, namely, an external membrane, analogous to the vitellary 

 membrane, but performing a different function; a granular membrane, 

 containing a thin fluid, corresponding to the immature yelk of a bird's 

 egg; and a vesicle in every respect analogous to the vesicle which 

 Purkinje found in the hen's egg, while still lodged in the ovary. The 

 author considers the granular membrane, proligerous disc, and gra- 

 nular fluid of the Graafian vesicle, as parts which are superadded, and 

 of which there is no trace within the capsule of the ovary of a bird. 



" Some Remarks on the difficulty of distinguishing certain genera 

 of Shells ; and on some Anomalies observed in the Habitations of cer- 

 tain species of Mollusca." By John Edward Gray, Esq., F.R.S. 



In opposition to the opinion of those geologists who consider that 

 all shells of the same form and character have been inhabited by one 

 genus of animals ; that all the species of a genus live in similar situa- 

 tions ; and that all the species of fossil shells, appearing from their cha- 

 racter to belong to some recent genus, have been formed by animals 

 which in their living state had the same habits as the most commonly 

 observed species of that genus, — the author proposes to show, first, that 

 shells having the appearance of belonging to the same natural genus 

 are sometimes inhabited by very different animals; and, secondly, 

 that some species of shell-bearing molluscous animals live in dif- 



