[96] 



OVUM. 



stance contains at first only fine granules ; but 

 in the second season of development there are 

 found mixed with these, especially towards 

 the surface, corpuscles of a somewhat larger 

 size, and these are gradually converted into the 

 quadrilateral tabular particles. The distinc- 

 tion of colour between the surface and the 

 deeper parts, and between the upper and lower 

 portions of the yolk, also now appears; but it 

 is not till the third season of development that, 

 along with a proportional enlargement of the 

 yolk, the darkest kind of pigment is deposited 

 among the corpuscles on the upper surface. 

 The gradual extension of this coloured layer 

 over a greater portion of the surface of u the yolk 

 from the upper towards the lower part has 

 already been stated. The extent of the 

 coloured portion marks in fact, in a great 

 measure, the proportion which the immedi- 

 ately germinal part of the yolk bears to that 

 not concerned in the first process of em- 

 bryonic development ; or it indicates at least 

 the extent of the yolk which is immediately 

 involved in the process of segmentation. 



The vitelline membrane, I have already said, 

 is absent during the early stages of develop- 

 ment of the ovum ; it appears to be present 

 in the third season, but I have not been able 

 to determine precisely its mode of origin. 

 Farther observations are still necessary to 

 ascertain whether, as in Mammalia and some 

 other animals, a zona is formed by the con- 

 densation of the outer part of the primitive 

 yolk-substance, or whether this membrane 

 proceeds from another source. From the 

 gradual flattening and disappearance of the 

 inner cells of the ovarian follicle, and the 

 close adhesion of their remains to the vitelline 

 membrane in the later stages, I am led to be- 

 lieve, that the covering with which the yolk 

 leaves the ovary may owe its origin to the 

 amalgamation of one or more layers of fused 

 or united cells of the tunica granulosa, or 

 to the union of these with the zona or primi- 

 tive vitelline membrane, should such exist. 



There is no true cellular yolk, but the 

 granular yolk is of proportionally large size ; 

 and if we are disposed to regard the yolk as 

 containing both a formative and nutritive part, 

 these are united or combined in a more close 

 manner than in the larger ova of ovipara. 

 The ova of Batrachia differ, on the other hand, 

 greatly from those of Mammalia in their re- 

 lation to the Graafian follicle ; more especially 

 in the fact of the ovum completely filling the 

 follicle, and the entire absence, excepting in 

 the epithelial lining, of fluid or other deposit 

 between that layer and the surface of the 

 ovum. The history of development shows 

 that the peculiar structure of the ovum of 

 Batrachia, as well as that of osseous fishes, has 

 some connection with the large proportion of 

 the yolk which becomes immediately germinal, 

 and with the comparatively early period of 

 advancement at which the young leave the 

 egg and assume an independent mode of life. 



Before concluding this account of the ovum 

 of the Amphibia, it will be proper to notice 

 the changes that have been observed in the 



germinal vesicle near the time of the discharge 

 of the ova, and in its descent through the 

 tubes till its exclusion. All observers are 

 agreed that the germinal vesicle is no longer 

 visible in the excluded ovum, whether fecun- 

 dation shall have occurred or not ; and the 

 solution or disappearance of this vesicle is 

 now looked upon, in these as well as in other 

 animals, as a natural concomitant of the 

 maturation of the ovum independently of 

 fecundation. The recent and very precise 

 observations of Newport* have shown, that 

 in a considerable number of the ova about to 

 leave the ovary but still situated within that 

 organ, the germinal vesicle has disappeared, 

 and that it is invariably gone in all those 

 which have passed into the abdominal cavity. 



Very shortly before disappearing, and when 

 the ovum is approaching maturity, a remark- 

 able change has been observed in the contents of 

 the germinal vesicle ; which is of great interest, 

 in consequence of its probable intimate rela- 

 tion to the process of segmentation and cell- 

 formation, which follow fecundation and are 

 the precursors of true embryonic develop- 

 ment. These changes have been described 

 first by Cramer, and afterwards by Newport; 

 the latter author, apparently, not having been 

 aware of the observations of the former. 



In early spring (February) Cramer f found 

 the fine granules into which the maculae of 

 the germinal vesicle had previously been re- 

 solved by multiplication, beginning to unite 

 together into heaps or small masses; and 

 somewhat later he found these masses to 

 become surrounded by a fine membrane or 

 envelope, giving them all the appearance of 

 small cells with a granular nucleus. There are 

 often several hundred such cells at this period 

 in the germinal vesicle of the brown frog, 

 varying slightly in size and shape. At a still 

 later period the greater part of the granular 

 nuclei or contents of these cells become dis- 

 solved, leaving only a few remaining in each ; 

 and finally these also disappear, so as to ren- 

 der the cells entirely clear. 



Now, all observers are agreed, that in the 

 yolk-substance of the ovarian ovum, previous 

 to the rupture of the germinal vesicle, there 

 are not to be perceived any other solid par- 

 ticles excepting those already mentioned, viz., 

 granules or heaps of granules, and the peculiar 

 quadrangular tables ; but many observers have 

 perceived that immediately after the disap- 

 pearance of the germinal vesicle, and during 

 the whole time previous to fecundation, as well 

 as after that change, the yolk-substance con- 

 tains, mixed with the darker corpuscles, other 

 clearer and spherical vesicular globules, some- 

 what larger than the tabular corpuscles. Vogt 

 described them as scattered through the whole 

 of the superficial yolk-substance in the Alytes 

 obstetricans, and Cramer pointed out that 

 these vesicular corpuscles are identical with 

 the cells which he had observed to be formed 

 in the germinal vesicle immediately before its 



* Researches, &c., 1st Series, p. 177. 

 t Muller's Archiv. 1848, p. 23. 



