OVUM. 



[133] 



mals, whatever may be the ultimate structure 

 of the yolk, the primitive yolk, or that which 

 is first formed, is invariably of the finely gra- 

 nular kind, the cellular or large corpuscular 

 yolk-substance is of later formation. These 

 two parts remain distinct from each other, and 

 the finely granular or formative yolk is that in 

 which the germinal vesicle is invariably im- 

 bedded. In those instances, such as the Bird, 

 Reptile, &c., in which the large cellular yolk 

 greatly preponderates over the formative yolk- 

 substance, the latter assumes in the later 

 stages of formation the shape of a flatfish 

 disc on one side of the greater mass of the 

 yolk, with the germinal vesicle placed in its 

 centre. 



The vitelline membrane presents some va- 

 rieties in structure, being in some animals 

 very delicate and homogeneous; in others, 

 as Mammalia, remarkably thick, tough, and 

 elastic, but without visible structure ; in a 

 third set, exhibiting peculiar structure, such 

 as the finely tubular perforations of the ex- 

 ternal membrane of the fishes' ovum, or the 

 radiated markings in the ova of Holothuria 

 or Cestoidea ; but in these last three in- 

 stances the vitelline membrane is probably 

 associated with additional layers of substance 

 derived from a different source from that 

 which forms the homogeneous membrane. 



A remarkable peculiarity has recently been 

 discovered in the enclosing membrane of the 

 ovarian ovum of some animals, in the aper- 

 ture or micropyle which has been observed 

 in osseous fishes, insects, some Crustacea*, 

 the Acephalous Mollusca, some Annelida, 

 Holothuria, and some other Echinodermata. 

 There seems reason to believe that a similar 

 aperture exists in the ovum of Batrachia and 

 Cephalopoda ; and it is very probable that it 

 may yet be discovered in other animals. At 

 the same time it is right to state that in 

 Mammalia and several other animals it has 

 been most carefully sought for without suc- 

 cess. This aperture appears to be designed 

 to admit the spermatozoa into the cavity of 

 the ovum, or into contact with the yolk-sub' 

 stance and germ, in those instances especially 

 in which the egg coverings are thick and 

 touiih, and fecundation is late of occurring. 



The relation of the ova to the ovaries or 

 organs in which they are produced, exhibits 

 considerable varieties in different animals. 

 1. The most common is that in which the 

 germs of the ova arise within minute close 

 follicles or vesicles, which are imbedded in 

 the more or less solid or loose stroma of the 

 ovary ; the follicle enlarging with the ovum 

 as its other parts are added till the period of 

 of maturity, when, periodically, the follicles 

 open for the escape of the ova. 2* In a 

 second form, as in Nematoid Worms and 

 Insects, the germs of the ova are produced 

 free in the upper part of an ovarian tube, 



* It has been inadvertently stated in a preceding 

 part of this article (p. 116.) that the micropyle had 

 not been observed in the ova of Crustacea, whereas 

 Meissner has ascertained its presence in that of 

 Gamraarus. (See his Memoir in Zeitsch. fur Wissen. 

 Zool. vol. v. p. 284.) 



and the yolk-substance, &c. are added gra- 

 dually as the egg germs descend through suc- 

 cessive portions of the tube : here no true de- 

 hiscence is necessary to allow of the escape 

 of the ova. 3. In a third form, as in Trema- 

 tode and Cestoid Entozoa, distinct organs are 

 provided for the formation of the ovigerms 

 and the yolk-substance, and these last are 

 brought together and combined into the sphe- 

 rical form of an ovum in another part of the 

 genital apparatus. 4. In the greater number 

 of animals the germs for each ovum appear 

 to arise singly, and the ova are thus isolated 

 from the first ; but it would appear that in 

 some animals these germs arise in groups, 

 perhaps by development from a common 

 germ, so that they are from the earliest period 

 connected together by pedicles. Yet, with 

 all these differences, there is to be perceived, 

 on the whole, a general similarity in the plan 

 of formation of the parts of the ovum itself in 

 different animals. This plan may be generally 

 stated as follows. 



The germinal vesicle is universally the first 

 part of the ovum which makes its appearance ; 

 it does not appear to be nucleated or to pos- 

 sess its macula from the first in all instances, 

 and this macula cannot therefore be regarded 

 as the centre of its formation. The germinal 

 vesicle is generally at first only a minute 

 point ; it soon enlarges, however, and either 

 possesses from the first, or at a very early 

 period acquires, its macula or nucleus. In 

 animals with the solid follicular ovary, each 

 follicle is occupied by a single ovum, which 

 begins within it as a minute germinal vesicle. 

 The delicate wall of the follicle is also per- 

 ceptible at the same time as the ovigerm ; in- 

 deed, there is reason to believe that it even 

 precedes the commencement of the formation 

 of the ovum, though this is a point not yet 

 fully determined. In those animals, on the 

 other hand, in which the ovary is tubular, the 

 ovigerms appear, in some instances at least, to 

 arise in groups within cells ; and it may be a 

 question whether these cells bear to the ovi- 

 germs arising within them the relation of the 

 ovarian follicles of solid or closed ovaries. 

 Whether this be so or not, that relation is in 

 most instances speedily changed, as the ova 

 soon become free, or, in others, are attached 

 by a pedicle to a common stalk. 



The wall of the ovarian follicle consists at 

 first of an extremely delicate vesicular mem- 

 brane, which is the same as that to which the 

 name of ovicapsule or ovisac has been given* 

 At a very early period, and while the ovum con- 

 sists of no more than the germinal vesicle, the 

 homogeneous wall of the follicle is lined with 

 a layer of flat cells somewhat analogous to 

 some forms of epithelium : this is the com- 

 mencement of the structure which in Mam- 

 malia afterwards forms the tunica granulosa, 

 and the fluid and cellular contents of the 

 Graafian follicle. It appears to have various 

 destinations in different animals. 



The second stage in the formation of the 

 ovum is the deposit of the vitelline substance 

 around the germinal vesicle. In most ani- 

 mals the yolk-substance, when it first begins 

 [K31 



