OVUM. 



[103] 



In D. T J 5 ", although the number of maculae has 

 greatly increased by endogenous multiplication, the 

 germinal vesicle has not now undergone an enlarge- 

 ment proportional to that of other parts of the egg 

 and ovisac : the granules of the yolk, especially to- 

 wards the surface, are much increased, and a narrow 

 clear marginal space on the surface now indicates 

 the commencement of the formation of a zona or 

 vitelline membrane. This appearance ia also 

 slightly perceptible in fig. c. 



The "dimensions of the several parts in these dif- 

 ferent specimens were as follows : 



A. B. C. D. 



Ovisac - - -0025 -004 -0056 -007 



Yolk -0042 -005 



Germinal vesicle - -001 -0016 -0025 -0026 



Maculae - - -00015 -00018 -00025 -0003 



he found the funnel-shaped aperture to re- 

 main for some time after the completion of 

 fecundation, and in none of the fishes he has 

 observed does he conceive the aperture of 

 the micropyle to be closed. 



The ova of osseous fishes appear to take 

 their origin within the rudimentary follicles 

 or ovisacs of the ovary mueh in the same 

 manner as those of the Batrachia. The ear- 

 liest part of the ovum that can be distinctly 

 seen within the follicle is a vesicle of about 

 half the diameter of the primitive follicle it- 

 self. A little later this vesicle is seen to be 

 surrounded with a dear, jelly-like substance, 

 in which some small dark granules are depo- 

 sited chiefly towards the surface of the vesi- 

 cle. There is as yet no enclosing membrane, 

 but the follicle is seen to be lined by a layer 

 of extremely delicate hyaline ceils, often dif- 

 ficultly perceptible. The earliest recognisable 

 part of the ovum, therefore, is the germinal 

 vesicle ; which, as in other animals, has soon 

 deposited round it the clear gelatinous base- 

 ment-substance of the yolk, in which the 

 opaque yolk granules soon make their appear- 

 ance. There is not at first any vitelline or 

 other membrane enclosing the primitive parts 

 of the egg, and indeed it is a considerable 

 time before any such membranes are formed. 

 The deposit of vitelline granules increases ra- 

 pidly, so as to give the yolk considerable opa- 

 city ; afterwards larger globules appear, and 

 seem to increase by endogenous multiplica- 

 tion. * The oil globules are at first small, 

 and equally diffused through the whole 

 yolk ; it is only in the later stages of for- 

 mation that they unite into fewer and larger 

 globules.f The granular or primitive yolk- 

 substance continues to surround more imme- 

 diately the germinal vesicle till the period 

 when this vesicle is ruptured, and is probably 

 spread over the germinal disc of the egg. Si- 

 milar granules also occupy, however, in a 

 layer the surface of this part of the egg pre- 

 vious to the rupture of the germinal vesicle ; 

 so that it is not probable that the germinal 

 disc owes its origin, as Coste states^:, entirely 

 to the effusion of the contents of the germinal 

 vesicle. 



* Lereboullet, loc. cit. 

 t Retzius, loc. cit. 



j Hist. ge'n. et part, du Develop?, des Corps 

 organ, torn. i. 



The ovum receives its firm porous mem- 

 brane while within the ovarian capsule, but 

 only in the latter part of the time of its forma- 

 tion. This membrane lies very close to the 

 inside of the ovisac, is at first comparatively 

 thin and destitute of apparent structure, and 

 gradually increases in thickness towards the 

 time of its approach to maturity. At the 

 same time a remarkably thin pellicle may be 

 distinguished close to the surface of the 

 granular yolk-substance, scarcely meriting the 

 name of membrane. As already remarked, it 

 is difficult to determine what is the true homo- 

 logical signification of these membranes. The 

 inner one might by some be regarded as a re- 

 presentative of the zona pellucida, or a conso- 

 lidated pellicle on the surface of the yolk, 

 though it must be admitted that Ransom's ob- 

 servation, that it follows the segmentation, is 

 opposed to this view, and makes it more 

 probable that it is only a part of the yolk 

 itself. The origin of the external porous 

 membrane I am inclined to connect rather 

 with the interior of the ovarian follicle ; but 

 whether by exudation from it, or by amalga- 

 mation of the innermost layer of epithelial 

 cells of the follicle, I have not yet been able 

 to determine. I am inclined to regard the 

 latter as most probable, and that this is the 

 true vitelline membrane. 



The manner in which the micropyle takes 

 its origin has not yet been ascertained. It 

 will afterwards be shown, that in a consider- 

 able proportion of those invertebrate animals 

 in which this aperture in the egg coverings 

 is found, it has existed from a very early 

 period, and proceeds from the remains of the 

 pedicle by which the ovum is originally con- 

 nected with the ovarian substance. Such a 

 pediculated connection has certainly not yet 

 been observed by most of those who have in- 

 vestigated the ovarian ovum of fishes.* 

 Rathke, indeed, observed the appearance of 

 the remains of a pedicle in the detached ova 

 of the Blennius viviparus -j- ; according to 

 Ransom the micropyle in the Pike is not 

 a depression, but projects from the surface 

 like a trumpet-shaped process; and in the 

 earliest stage of development of the ovarian 

 ovum of Trigla hirundo, according to Ley- 

 dig J, the shape is somewhat pyriform or 

 pediculated, in the same manner as in some 

 of the invertebrate animals. 



On the other hand, Ransom expressly 

 states that he has never been able to observe 

 the slightest connection in Gasterosteus be- 

 tween the pedicle of the ovum by which 

 it is attached to the ovary, and the mi- 

 cropyle. This aperture he says is always 

 situated at that side of the ovum towards 

 which the germinal vesicle and the germinal 

 disc are placed ; but these parts have no 

 regular connection with the pedicle. The pe- 



* The pedicle here spoken of is not that of the 

 ovarian capsule containing the ovum, but of the 

 ovum itself within the capsule, 



\ Abhandlung. zur Entwick. part. ii. p. 4. 



t MUller's Archiv. for 1854, p. 376. fig. 6. 



