OVUM. 



71 



been discharged from the capsule in a few 

 hours, we may find some difficulty in isolating 

 the vesicle of Purkinje from the granular disc ; 

 for, by this time, the vesicle has become 

 flaccid, weak, and flattened down, and has 

 begun to be softened and dissolved, prepara- 

 tory to its complete disappearance, which 

 generally occurs about the time when the 

 stigma of the capsule opens to allow of the 

 escape of the yolk into the infundibulum 

 which embraces it. But, in all the other 

 yolks down to those of -J^ of an inch in dia- 

 meter, it is quite easy to break up the granular 

 disc with needle points, and to preserve the 

 vesicle uninjured. We may then free it entirely 

 from adhering granules, and cause it to roll 

 along in the fluid in which it is immersed, or 

 on a plate of glass ; and we may perceive that 

 it is a simple membranous vesicle filled with 

 fluid, and without any very obvious granules or 

 nuclei. In the perfectly fresh state,the contents 

 of the vesicleare almost limpid, exhibiting only 

 a slight turbidity scarcely amounting to a 

 granular deposit, provided it has been placed 

 in a medium which does not change its ap- 

 pearance ; but, if it is allowed to remain a 

 short time in water, and still more if it is im- 

 mersed in fluids which coagulate albumen, its 

 interior speedily assumes a minutely granular 

 aspect. The external wall of the vesicle then 

 separates somewhat from the spherical gra- 

 nular mass within ; and I have sometimes seen 

 (as represented in fg. 51, F) a considerable 

 condensation of the granular mass, so as to 

 leave a large clear space between it and the 

 external vesicle, and give it very much the ap- 

 pearance of the yolk mass in the ova of some 

 small animals within the vitelline membrane. 

 This change seems to be a combined effect of 

 the condensation of the granular mass and 

 the imbibition of fluid by the external vesicle. 

 In the earlier ovula this rounded molecular 

 mass is of proportionately smaller size ; and 

 although it differs very much from the smaller 

 nucleus or macula contained in the germinal 

 vesicle of the ova of many other animals, there 

 can be little doubt that it is derived from this 

 structure, as will appear from what is hereafter 

 said of the progress of its development. 



When the yolk has passed into the ovi- 

 duct, and, in most instances, probably even 

 sooner, or when it has entered the infundibulum, 

 the germinal vesicle has entirely disappeared. 

 Sometimes it is already gone before the open- 

 ing of the ovarian capsule. The cicatricula 

 then presents an irregularly broken appear- 

 ance in consequence of the want of support 

 from the wall of the vesicle, and the dif- 

 fusion of the contents of the vesicle over 

 the surface of the proligerous disc. The 

 solution of the wall of the vesicle is probably 

 a gradual process connected with the state of 

 complete maturation of the ovule. It occurs, 

 as is well known, in the unfecundated as well 

 as in the fecundated egg, and cannot, there- 

 fore, in itself, be dependent on the action of 

 the spermatozoa; neither is it altogether 

 caused by the mechanical pressure to which 

 the yolk is subjected in issuing from the 



ovarian capsule, nor by the pressure of the 

 oviduct itself; for it usually begins, and is 

 sometimes completed before these causes can 

 operate. 



The diffusion of the germinal substance from 

 the vesicle (which in the fowl must have 

 already received the spermatic influence in 

 the ovary) has the effect thus of mingling 

 with the remainder of the cicatricula, a ma- 

 terial which, it can scarcely be doubted, ex- 

 erts some immediate influence in inducing the 

 change of segmentation and subsequent pro- 

 cess of organisation by which the blastoderm 

 is produced. 



Microscopic structure of the ovum. The in- 

 vestigation of the microscopic structure of the 

 yolk is attended with considerable difficulty, 

 in consequence both of the variety and the deli- 

 cacy of the organised elements of which it 

 consists. The following parts require our 

 separate attention viz., 1st. The yellow or 

 external yolk substance ; 2nd, the substance 

 of the cavity and canal ; 3rd, that of the cica- 

 tricula and cumulus ; 4-th, the vitelline mem- 

 brane. We shall consider these both in the 

 laid egg and in the ovarian capsule. 



1. From the effect of boiling the yolk, 

 every one is familiar with the fact that its 

 yellow substance is coarsely granular ; but the 

 exact nature of the small bodies giving this 

 granular structure has not been equally well 

 understood. The examination of this sub- 

 stance with a microscope of moderate magni- 

 fying power in a newly laid egg, shows that 

 almost all of the deeply coloured part of the 

 yolk consists of spherical corpuscles of con- 

 siderable size, so closely set together that 

 sthey are mutually compressed ; and thus, 

 when the yolk has been hardened by boil- 

 ing, the substance of the corpuscles being 

 coagulated by heat, they present polyhedral 

 forms ; but when diffused in fluid in the un- 

 boiled state, they are all nearly or quite sphe- 

 rical. The size of these corpuscles varies 

 between T i^- and -g^ of an inch ; but the 

 greater number of them are more near -^^ or 

 2 ^Q. Some have described the yolk cor- 

 puscles as floating in a fluid ; and no doubt 

 in the earlier condition of the yolk, a consi- 

 derable quantity of fluid exists, but in the more 

 advanced condition the amount of mutual com- 

 pression they exert when coagulated is suf- 

 ficient to show that its quantity must be 

 very small indeed. Those who have de- 

 scribed the yolk substance as mainly consist- 

 ing of a fluid holding in suspension a quan- 

 tity of extremely minute granules or molecu- 

 les, together'with some larger corpuscles, have 

 probably been misled, by making an examina- 

 tion of the yolk when not perfectly fresh, and 

 when the larger corpuscles have been in part 

 broken up, and thus resolved into the granular 

 fluid of which they consist. There is no 

 doubt that in birds, and in all the large-yolked 

 animals, the deeply coloured vitelline sub- 

 stance, which, in fact, forms the great mass af 

 these ova, consists almost entirely of the large 

 and usually spherical corpuscles just now 

 noticed. In some animals the form is not 



