74, 



OVUM. 



rally received this name in the fowl's egg, 

 and which I have hitherto regarded as corre- 

 sponding with the immediate membranous in- 

 vestment of the yolk (zona pellucida) in 

 mammalia, and in all animals, constitutes, 

 both in the mature ovarian yolk and in the 

 laid egg, an entire thin transparent covering 

 of the yolk substance, without any aperture 

 that has been discovered in it at any time ; 

 delicate and easily torn, but yet of such con- 

 sistence that under water any portion of it may 

 easily be removed and examined. In the egg 

 which has passed through the oviduct, the vi- 

 telline membrane floats free from the cicatri- 

 cula and surface of the yolk substance ; but, 

 so long as it remains in the ovarian capsule, 

 these parts cohere somewhat together j so 

 that, in general, on removing a part of the yolk 

 membrane, a more or less complete lining of 

 the nucleated or outermost layer of yolk cells 

 comes away with it. The microscopic exami- 

 nation of this membrane in the fully formed 

 yolk does not, as already stated, show any 

 very distinct structure beyond an obscure fi- 

 brillar and molecular marking, of such fineness, 

 indeed, as to require a high magnifying power 

 (500 to 600 diameters) to bring it into view ; 

 and in many parts the membrane appears per- 

 fectly homogeneous. In the earlier stages of 

 the yolk's growth, however, we shall see that 

 this membrane is not to be distinguished from 

 the layer of closely set nucleated cells, the outer- 

 most part of which appears to become fused 

 together into the membrane as the yolks ad- 

 vance to maturity (see fig. 53, K vm). We shall 

 presently see that the vesicular envelope which 

 is generally termed the yolk membrane in the 

 bird's egg, and in the ova of all animals pos- 

 sessing the large yolks, is probably a different 

 structure from the perfectly homogeneous ve- 

 sicle which in many other animals arises at a 

 much earlier period of the growth of the ovule, 

 and remains in them as the external covering 

 of the yolk to the end. 



Early condition and first formation of the 

 ovarian ovum in birds. It has already been 

 stated that the ovula exist at a very early 

 period of life in the female bird ; constituting 

 in their earliest undeveloped condition minute 

 cells closely surrounded by the simple vesi- 

 cular capsules and the solid substance of the 

 ovary, which at [this period has not lost its 

 primitive compact form. As the bird ap- 

 proaches maturity, a considerable number of 

 the ovula situated nearest the surface in- 

 creasing in size make an advance in their 

 structure by undergoing certain changes which 

 will immediately be more particularly adverted 

 to. Having attained various sizes from $ to 

 - of an inch, they project slightly as rounded 

 bodies from the surface of the ovary, and 

 remain in this condition till the approach 

 of the breeding season, when some of them 

 destined to reach their full state of develop- 

 ment, are at last discharged from their ovarian 

 capsules. A much greater number, however, 

 must remain in the undeveloped condition, 

 awaiting future seasons of evolution ; and a 

 very considerable proportion of the whole 

 germs of the ovary rather pass through a 



retrograde process and gradually disappear 

 without having attained to any considerable 

 size. 



Of the smaller or undeveloped ova, such as 

 those of less than ^ of an inch in dia- 

 meter, some are of a dull whitish or milky 

 colour, the deeper-coloured external yolk 

 substance not having been yet formed, and the 

 yolk substance consisting almost entirely of 

 small spherules or globules, not of true cells or 

 of the granular corpuscles which appear at a 

 later stage. Those between -^ and of an 

 inch are for the most part of a lighter yellow 

 than the larger ovula ; but above the latter 

 size the colour has attained nearly its full in- 

 tensity from the deposit externally of the 

 deep-coloured yolk substance. 



In all the ovula above -J^ of an inch it is 

 easy to see the germinal vesicle situated on 

 the surface of the yolk, when the capsule is 

 opened, embedded in a more opaque and 

 compact layer of substance which repre- 

 sents the discus proligerus, extending at this 

 period nearly over the whole surface of the 

 yolk. But in those less than ^ or ^ of an 

 inch, the vesicle is not to be seen on the 

 surface. On carefully opening or breaking up 

 the substance of the yolk, the vesicle is easily 

 found in the softer internal substance which 

 flows out from the centre. From the central 

 part of the small ovule, the vesicle appears 

 gradually to pass outwards towards a deter- 

 minate part of the surface, making its way 

 through the proligerous layer or primitive 

 yolk granules ; and thus, in examining ovula 

 at this stage, I have been able to perceive 

 occasionally that the vesicle was situated ,in 

 a more or less deep depression on the inner 

 surface of that layer, which therefore must 

 be perforated, as it were, by the vesicle in 

 its passage towards the surface. The sub- 

 stance of the disc afterwards collects round 

 the vesicle internally, and is accumulated in 

 greater quantity (cumulus) in that situation. 

 This change of place of the germinal vesicle 

 from the centre or interior to the surface of 

 the yolk in the progress of development of 

 the ovula occurs in some degree throughout 

 the animal kingdom ; but it is especially re- 

 markable in the eggs of birds and other 

 animals with large yolks, in consequence of the 

 peculiar connection of the vesicle with the 

 proligerous disc. In the batrachia also, the 

 change is very obvious, and the progress of 

 the vesicle outwards has been well described 

 by Von Baer and others. In this latter class 

 of animals the proligerous layer covers a much 

 greater part, or indeed in most of them nearly 

 the whole of the yolk ; but the germinal 

 vesicle occupies always a determinate place in 

 the centre of the layer ; showing that the 

 development of the various parts of the 

 ovum proceeds from the first with a fixed 

 relation of position between the germinal ve- 

 sicle and other parts. 



In birds, as in all other animals, the ger- 

 minal vesicle, which we shall see is the fun- 

 damental part of the ovum, is proportion- 

 ally large in the earlier stage of growth of 

 the ovule, being at the first from a fourth to a 



