OVUM. 



ash-coloured. In the land-newt, which is oyo- 

 viviparous, the yolk is of considerable size, 

 and of a dark yellow, approaching to orange. 



In osseous fishes, which are almost all 

 oviparous, the ovule receives, apparently in 

 the ovarian capsule itself, before leaving that 

 cavity, an external covering (or chorion) of 

 considerable firmness. This membrane ap- 

 pears to consist of a substance deposited on 

 the external surface of the vitelline membrane, 

 and becomes coagulated under the action of 

 water ; so that its density increases greatly 

 after the ova are deposited, while it is sepa- 

 rated at the same time from the yolk by 

 the imbibition of water. The ova are in 

 spawning either deposited separately, or are 

 united in chains or bundles, and in some 

 less common examples* in peculiar nida- 

 mental structures, more after the manner 

 of some of the mollusca. The structure of 

 the ovarian ovule, or yolk, and its relation 

 to the germ, differs somewhat from that of 

 the batrachia ; for while in the latter animals 

 the yolk substance consists of granules and 

 cells of nearly uniform size, and the germinal 

 layer covers the greater part of the surface, 

 in osseous fishes this layer is more circum- 

 scribed, riot extending at first over more than 

 a third, or, at most, a half of the yolk, and 

 the remainder of the yolk, which contains a 

 much greater quantity of transparent fluid than 

 in most other vertebrate animals, presents al- 

 most invariably a peculiar heap or mass of 

 large oil globules, which float to the upper 

 part of the fluid below the germinal layer.f 



The minute ovula of mammalia, when they 

 have reached maturity in the Graafian capsules 

 of the ovary, are nearly spherical bodies, of 

 from T ij 7 to -^ of an inch in diameter, and 

 consist of a mass of finely granular yolk sub- 

 stance, more loose in the interior and more 

 dense towards the surface, and enclosed in a 

 thick firm and transparent vesicular envelope, 

 the vitelline membrane, or so-called zona 

 pellucida. While still within the Graafian 

 capsules, they occupy a situation near the 

 most projecting part of the capsule, or 

 towards the external surface of the ovary, 

 being there imbedded in a layer of granular 

 cells, the discus proligerus of Von Baer, which 

 lines the ovicapsule, and lies on the exterior 

 of the clear coagulable fluid with which this 

 capsule is filled. A portion of this lining 

 membrane of granular cells, remains adherent 

 to the ovum after it leaves theGraafian capsule, 

 and has passed into the Fallopian tube ; but as 

 it descends towards the uterus, these cells gra- 

 dually loosen and fall away from the surface 

 of the ovum, the zona pellucida or vitelline 

 membrane of which is thus finally left free. 

 In the farther progress of its descent, there 

 is formed, in some mammalia at least (rabbit), 



* As Gohius. See Prof. Owen's Lectures on the 

 Compar. Anat. and Physiol. of Vertebrated Animals, 

 part i. p. 304. A. Hancock on the Nidification of 

 the Gasterosteus aculeatus, &c., in Annals of Natu- 

 ral History, Oct. 1852. 



t See a "paper by Dr. Davy on the chemical pro- 

 perties of the vitellus of Osseous fishes in Trans. 

 Roy. Soc. for 1851. 



on the surface of the zona by a new deposit, 

 in others, perhaps, by conversion of the zona 

 itself, the external membrane of the ovum, 

 which at a later stage constitutes the chorion. 

 But, in accordance with the destination of the 

 ovum in this tribe of animals for true utero- 

 gestation, this external membrane has then no 

 longer the character of mere inactive limita- 

 tion of the exterior of the ovum, or defence 

 from injury, which belongs to it in the lower 

 animals'; but it becomes an organised and 

 growing texture of active functions, which is 

 the more immediate means of uniting organi- 

 cally the blood-vessels of the mother and 

 foetus, in such a manner as to allow of the 

 transmission of nourishment from the one 

 to the other. 



Varieties of form of the ova among the 

 invertebrata are too numerous to allow of 

 their being described in this place. In the 

 greater number, an external envelope, besides 

 the vitelline membrane, exists ; but it must 

 be admitted, that there are some in which 

 these two coverings cannot be distinguished. 

 In some, as in insects, arachnida, polypes, &c., 

 the chorion, or outer surface, presents pecu- 

 liar markings, ridges, tubercles, or long spines, 

 and is strong and opaque ; in others, it is 



Fig. 34*. 



Ovum of Cristatella mucedo. 



(From Turpin, Annal. des Scien. Nat. 1837. torn, 

 vii.) Showing peculiar spinous projections from the 

 outer shell. 



smooth, delicate, and transparent, so as to 

 allow the whole internal structure of the 

 ovum to be seen through it, and thus to 

 afford most favourable opportunities of wit- 

 nessing the early changes of development. 

 In most of the invertebrata the germinal 

 part of the yolk covers the whole, or a con- 

 siderable part, of its surface ; they present, 

 however, great varieties of colour and struc- 

 ture, and may, probably, belong to various 

 modifications of the second and third groups 

 before distinguished. 



It does not appear that any essential dif- 

 ference has yet been observed in the structure 

 of the ova of those animals which are subject 

 to alternate generation, and those of animals 

 in which the adult form is directly developed 

 from the ovum. 



3. Of the ovary in general as the forma- 

 tive organ for the ova of animals. 



