OVUM. 



[99] 



tides become more and more circumscribed 

 in a layer on one side of the egg as it ap- 

 proaches maturity, so as to form a germinal 

 disc ; and this occurs independently of fecun- 

 dation. 



The germinal vesicle is not easily perceived 

 in the ovarian ovum when it has attained 

 some size. This proceeds in part from its ex- 

 treme delicacy and transparency, and also from 

 the opacity of the granules of the yolk within 

 which it is situated. But it is to be observed 

 also, that it disappears proportionally sooner 

 than in other vertebrate animals. It is of con- 

 siderable size in proportion to the rest of the 

 ovum, having a diameter not unfrequentlyof -^ 

 or g\/' m ova f sV- It ' ls never to be found in 

 ova that have left their capsules in the ovary ; 

 and according to Lereboullet's* observations 

 in the pike and perch, it has already disap- 

 peared for a considerable time before it attains 

 complete maturity. In the earlier stages of 

 the growth of the ovum the germinal vesicle 

 contains numerous distinct maculae ; but in 

 the progress of development these multiply 

 to a great degree, so that the vesicle is at last 

 completely filled with fine clear cells, or bril- 

 liant vesicles, and extremely minute granules. 

 When the vesicle bursts, its contents are dis- 

 persed over the yolk, and very probably are 

 mingled or combined with the layer of germi- 

 nal granules ; but it is not probable, as Lere- 

 boullet supposes, that the whole of the forma- 

 tive layer of the germ (afterwards undergoing 

 segmentation) is produced from the effused 

 contents of the germinal vesicle. This mul- 

 tiplication of the maculae and filling of the 

 germinal vesicle with fine cells appears to be 

 of an analogous kind to that which has been 

 described by Vogt and Newport in the Ba- 

 trachia ; and it seems not improbable that in 

 both classes of animals the dispersed maculae 

 may in some way or other, not yet fully ascer- 

 tained, contribute to the origin and develop- 

 ment of the blastodermic cells in the forma- 

 tion of which the process of segmentation re- 

 sults. It appears certain at least that, after 

 the disappearance of the germinal vesicle and 

 the dispersion of its contents, a marked change 

 takes place in the disposition of the germinal 

 part of the egg by its granular disc or layer 

 becoming more circumscribed and distinct ; 

 and, as Lereboullet supposes, it may then be 

 mingled with the brilliant points which pro- 

 ceed from the contents of the germinal ve- 

 sicle. 



The process of segmentation, into the de- 

 scription of which it is not intended at pre- 

 sent to enter, is co-extensive with the granu- 

 lar layer or germinal disc of the ovum. The 

 larger yolk globules and the fat cells are not 

 immediately concerned in this process.^ 



* Resume* d'un Travail sur I'Embryogenie da 

 Brochet, de la Perche, et de PEcrevisse, in Annal. 

 des Scien. Xat. 1854, torn. i. p. 237. et seq. 



t M. Coste (Comptes rendus, 1850, vol. xxx. 

 p. 692.) has described the germinal disc as being 

 formed only after fecundation ; but from the obser- 

 vations of Vogt, Aubert, Lereboullet, and Ransom, 

 it is ascertained that it exists previously. 



The membranes of the ripe ovarian ovum 

 of osseous fishes have been described by most 

 recent observers as two in number; viz. 1st, 

 the external tough membrane which some 

 have called chorion or shell-membrane, and 

 others vitelline membrane, which possesses a 

 peculiar structure, hereafter to be described 

 more particularly ; and, 2nd, an extremely 

 delicate film of membrane lying close to the 

 yolk-substance and destitute of visible struc- 

 ture. The latter of these membranes is 

 just discernible in the ovarian egg at the later 

 periods of its growth ; but in ova of two 

 thirds their full size I have failed to perceive 

 it. Dr. Ransom has observed, that in the 

 Stickleback, Gasterosteus aculeatus,this mem- 

 brane becomes more distinctly marked off* 

 from the substance of the yolk subsequent to 

 impregnation, and that it follows the inflec- 

 tions of the surface of that substance during 

 segmentation ; from which he infers, that 

 it is not to be compared with the vitelline 

 membrane as heretofore described by authors 

 in the ova of other animals. The observa- 

 tions which have been obligingly communi- 

 cated to me by Dr. Ransom leave no doubt 

 as to the existence of this inner membrane, 

 and have shown the new and interesting fact 

 that it is possessed of some vital contractile 

 power. It seems probable that it proceeds 

 from a consolidation of the outermost layer 

 of the basement or clear substance of the 

 yolk, in a manner somewhat analogous to the 

 zona pellucida. But I refrain from saying 

 more of it at present, as Dr. Ransom will ere 

 long probably communicate his observations 

 to the public in detail. 



The external membrane of the Fish's egg 

 which has been deposited in spawning or has 

 been extracted from the ovary when approach- 

 ing maturity, presents a remarkably well de- 

 fined line internally, and is also generally 

 smooth on its outer surface. In some fishes, 

 however, as the perch, it is covered externally 

 with villous, reticular, or other appendages, 

 which serve to connect the ova in masses or 

 strings, in the same manner as occurs with the 

 albuminous matter added to the ova of some 

 Batrachia, but in a less degree. In other in- 

 stances, as the Stickleback, these villi or project- 

 ing processes are limited to one portion of the 

 exterior. This membrane possesses consider- 

 able thickness and tenacity, and usually gives 

 the ovum a nearly regular spherical form when 

 imbibition is complete, as is the case after 

 impregnation. Previous to that change, how- 

 ever, the outer covering of the Fish's egg is 

 more yielding, and possesses so little elasticity, 

 that it usually retains dimples or impressions 

 made upon it from without. Two peculiarities 

 of structure have been observed in this mem- 

 brane which both merit farther attention, and 

 one of which is of great interest. 



The first of these to which I will refer is 

 the dotted or porous structure of the external 

 membrane. Von Baer* had remarked that the 

 external membrane of the ova of the Cyprinus 



* Entwickelungsgesch. der Fische, Leipzig, 1835. 

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