80 



OVUM. 



great delicacy, and is perceived only with diffi- 

 culty. The yolk substance is in great part com- 

 posed of peculiar angular corpuscles, sometimes 

 flat and tabular, at other times cubical, octahe- 

 dral, and of other forms. It has been stated 

 by Leydig * that in the plagiostomatous fishes 

 there is no vitelline disc or cicatricula ; but 

 this does not agree with my observations in 

 the common skate fish, in the ova of which I 

 have always observed a very distinct flat disc 

 about three-eighths of an inch in diameter, 

 of a lighter colour than the surrounding part 

 of the yolk, and situated on the surface which 

 naturally floats uppermost. The germinal 

 vesicle, which is larger than that of birds, 

 sometimes ^ f an mcn m diameter, is ex- 

 tremely delicate and very easily ruptured. 

 Its nucleus or macula is originally single, but 

 becomes afterwards subdivided or multiple 

 and diffused. 



The ova of the cephalopoda, which may, 

 according to most embryologists, be included 

 in this group, are of considerable size. They 

 possess a germinal disc, and the segmentation 

 of the yolk is partial or is confined to that 

 disc, as in the animals previously mentioned. f 

 The large yolk corpuscles are spherical 

 masses of united oil granules, and like the 

 most of those of the bird's egg without any 

 true enclosing vesicle. The germinal vesicle 

 is of considerable size, and has subdivided 

 maculae. The whole are enveloped by a fine 

 structureless vitelline membrane. In the 

 course of the formation of the ovum, the ex- 

 ternal part of it undergoes a remarkable 

 change in being folded inwards in numerous 

 deep grooves, which almost reach the centre 

 of the yolk. This folded structure is in the 

 smaller species again effaced before the ova 

 reach maturity, but in the larger species it 

 appears to remain, and to be the cause of a 

 peculiar reticulated marking over the surface 

 of the ova. 



In regard to other classes of the inverte- 

 brate animals of which the ova may appear to 

 belong to this group, I shall have occasion 

 hereafter to state some additional facts, in 

 explanation of the manner in which in some 

 of them the granular and cellular elements of 

 the yolk are occasionally combined. 



From the foregoing considerations, then, it 

 appears that in ova of the first group as dis- 

 tinguished in this treatise, the germinal 

 vesicle is at once the first formed and the 

 fundamental part of che ovum around 

 which the rest of the parts are deposited. It 

 alone, therefore, is strictly to be regarded as 

 a simple or primary cell. It may aptly be 

 called the Germ-cell. In birds its nucleus or 

 macula is originally simple, but at a very 

 early period it undergoes subdivision into 

 nucleoli or maculae, so numerous and minute 



* Beitrage zur Mikroskop. Anat. der Rochen 

 und Ha'ien, p. 87., a work which I quote only from 

 Leuckart. . 



t We owe this interesting discovery to Kolliker. 

 See his work on the Development of the Cephalo- 

 poda. Zurich, 1844. 



as at last scarcely to be recognisable, and thus 

 the germinal vesicle loses in such animals 

 somewhat the characteristic cellular struc- 

 ture which it presents in many others. Next, 

 by aggregation of the primitive yolk substance 

 round the germ-cell, and by the gradual con- 

 solidation of a clear film on the outermost 

 part of the albuminous matrix of this sub- 

 stance into the form of an enclosing vesicle, 

 there is produced the primitive ovum, a secon- 

 dary cell, in which the zona pellucida consti- 

 tutes the cell wall, the oil granules and albu- 

 minous fluid of the yolk substance the con- 

 tents, and of which the germinal vesicle is 

 now the nucleus. And lastly, if it is consi- 

 dered desirable to extend the application of 

 the term cell also to the mature state of the 

 ovarian egg of the bird, we may perhaps still 

 regard that organism as a tertiary cell (in the 

 ascending series) formed by the superposition 

 of new parts, some of which are themselves 

 cellular in their origin, round the primitive 

 ovum, the whole being at last enclosed and 

 moulded in the vesicular form by the external 

 vitelline membrane ; a peculiarity of this for- 

 mation being that in the meantime the wall 

 of the secondary cell or primitive ovum has 

 disappeared. But, notwithstanding the sphe- 

 rical form, the isolated position, and the simple 

 structure of the parts which compose the 

 completed ovarian ovum of birds, doubts may 

 fairly be entertained as to the propriety of 

 bringing such complex organisms under the 

 designation of cell as now generally em- 

 ployed. 



$ 5. More detailed description of ova be- 

 longing to the second group, or with small 

 granular yolk and complete segmentation. 



If the foregoing views are correct, it appears 

 that, while the ova of birds and other animals 

 of the first group contain within the vitelline 

 membrane the whole product of formation 

 which belongs to the ovarian capsule, the ova 

 of mammalia and animals of the second group 

 comprehend only those parts of the contents 

 of theGraafian follicle which are formed within 

 it at the earliest period viz., the germinal 

 vesicle and the primitive yolk substance with 

 its limiting zona. In the Graafian follicle of 

 mammalia, there is likewise produced a large 

 proportional quantity of cellular elements and 

 albuminous fluid, the tunica and substantia 

 granulosa, &c., in a part of which (cumulus 

 proligerus), the ovum comes to be imbedded 

 in the progress of their formation ; and these 

 superadded contents of the Graafian follicle 

 escape along with the ovum when it leaves 

 the ovary, but do not apparently maintain 

 any long or constant connection with it. 



In the ova of those invertebrate animals 

 which belong to this group, the conditions of 

 formation do not admit of their comparison 

 with those of mammalia ; but they agree with 

 them in so far that the first germ-cell is en- 

 closed by a finely granular yolk, and that the 

 vesicular envelope of the ovum generally cor- 

 responds to the zona formed by consolidation 

 on the surface of the primitive yolk. 



