Fecundation of the Ovum. 309 



It must not be supposed that the shagreened envelope of the 

 Fishes' egg is the analogue of the chorion of the Mammalia, or 

 of the shell, or the shell-membrane of Birds. In fact, the chorion 

 and the membrane of the shell are not formed until after fecun- 

 dation, so tliat it would be useless to seek in them for a micro- 

 pyle; the former does not exist in the Graafian vesicles of the; 

 Mammalia, nor the latter in the ovisacs of Birds. The shagreened 

 membrane of Fishes, or capsular envelope, exists in the ovarian 

 follicles, and consequently before fecundation, so that the sper- 

 matozoa must traverse it to effect that operation. It is there- 

 fore furnished with a micropyle, and must be compared with 

 the zona pcllucida of the Mammalia and the vitelline membrane 

 of Birds. The zona pellucida of the mammalian ovum, the 

 vitelline membrane of Birds' eggs, the shagreened membrane of 

 those of Fishes, the envelope with a crystalline structure of the 

 ova of the Holothurice, the cortical membrane of those of the 

 Naiades, the vitelline membrane of those of Mermis and Ascaris, 

 and the external envelope of the eggs of Insects and of Gum- 

 marus pulex, are therefore one and the same thing, and may be 

 designated the membrane of the mia-opijle*. It is true that the 

 ova of Gasterosteus and those of other fishes have an apparently 

 homogeneous membrane beneath the membrane of the micropyle, 

 and, to establish a complete analogy in the ova of other animals, 

 it would be necessary to ascertain the existence of this second 

 membrane in them. Barry f has already asserted, that he ob- 

 served a membrane between the zona pellucida and the vitellus 

 in the Mammalia. The Insects possess a second membrane, 

 furnished like the first with a micropyle. Miillerf speaks of 

 a membrane which immediately envelopes the vitellus in the 

 Holothurice, of which, however, Leuckart§ denies the existence. 

 Keber || asserted that in the Naiades, besides the cortical mem- 

 brane, he recognized a vitelline membrane, and even a membrane 



* It is true that we do not yet positively know whether the zona pellu- 

 cida possesses a micropyle, although Barry should have seen it, and 

 Meissner once ascertained the presence in it of an 0])eniug which did not 

 appear to be torn. There remain the Reptiles, of which the vitelline 

 membrane, to judge from Newport's observations on the ova of the Frog, 

 must be permeated in all parts by the spermatozoa. But this does not 

 appear to be a general rule amongst the Reptiles, nor even amongst the 

 Batrachia, for, according to an unpublished discovery of Meissner's, the 

 ova of the common Tree Frog {Hyla arborea) appear to possess a mi- 

 cropyle. 



t Researches in Embrj'ology, Third Series. Phil. Trans. 1840. It is 

 his " proper membrane of the substance by which the germinal vesicle is 

 surrounded." 



X Echinodermen, 4te Abhandlung, 1850. 



§ Zusatz, &c. 



II Ueber den Eintritt der Samenzellen in das Ei. 1853. 



