[98] 



OVUM. 



Salamandrina in regard to the structure and 

 formation of their ova. 



Osseous Fishes. The ovarian ova of Os- 

 seous Fishes, while they bear a general resem- 

 blance to those of Vertebrata, among which 

 they come nearest to those of Batrachia, are 

 distinguished by several marked peculiarities. 

 They are of middle size, and possess a strong 

 external covering formed within the ovary. 

 The yolk-substance contains several kinds of 

 elements ; and the germinal vesicle is of con- 

 siderable size. The external membrane is 

 thick, strong, and elastic, and of a peculiar 

 porous structure. The yolk-substance con- 

 tains a large quantity of clear fluid, in which 

 the albuminous granules and yolk corpuscles 

 and the oil globules are suspended, the latter 

 usually of large size and few in number ; the 

 germinal layer or disc is limited to a part 

 of the yolk, varying in size from about a sixth 

 to a half of the circumference, and the process 

 of segmentation in this part after fecundation 

 is consequently more limited than in Mam- 



Fig. 66 *. 



Ovum of the Gasterosteus at the time of impregnation. 

 (From Ransom. ) 



A. An ovum of the Stickleback eight or ten 

 minutes after impregnation, showing the clear re- 

 spiratory space formed immediately upon the access 

 of spermatozoa between the external membrane 

 and the surface of the yolk. Towards the upper 

 part of the figure the situation of the micropyle is 

 indicated by the small projections in the external 

 membrane ; towards the same or upper part of the 

 yolk the germinal disc or layer is easily distinguished 

 from the clearer part of the yolk ; and in the middle 

 a few large coloured oil globules. 



B. The same ovum about three minutes after im- 

 pregnation, showing somewhat in profile the funnel 

 of the micropyle descending into a depression on the 

 upper surface of the germinal part of the egg. In 

 consequence of impregnation, however, the funnel of 

 the micropyle has begun to rise out of the hollow, 

 and the respiratory space to be formed by the 

 separation of the external membrane from the sur- 

 face of the yolk. 



malia and most Batrachia ; but more extended 

 than in birds or scaly reptiles. The germinal 

 vesicle contains subdivided or multiple ma- 

 culae. I now proceed to give a few details 

 with respect to these several parts of the 

 ovum. 



The yolk-mass or yolk-substance consists, 

 in the more mature ovarian ova, of three parts : 

 viz., the clear fluid, which is in great abun- 

 dance and occupies chiefly the centre and the 

 lower part of the ovum; the superficial layer 

 of fine granules, with the vesicular corpuscles; 

 and the large oily globules, which from their 

 less specific gravity are usually situated to- 

 wards the surface and on the upper side. In 

 a number of fishes the clear fluid, which has an 

 acid reaction, becomes immediately turbid or 

 quite thick by the deposit of granular substance 

 when water is added to it. This change is 

 very apparent in the ova of the trout or sal- 

 mon, which, when placed in water, retain their 

 natural clearness and colour so long only as the 

 coverings are entire ; but immediately on their 

 being divided so as to allow of the action of 

 water on the contents, the whole yolk is sudden- 

 ly precipitated as a thick and somewhat tena- 

 cious granular mass. The albuminous matter 

 which surrounds ova that have been spawned 

 has an alkaline reaction. It is an interesting 

 fact, that in these ova, when imbibition of 

 water takes place as a consequence of fecun- 

 dation, no precipitate follows ; but that in 

 unfecundated ova left for some time in 

 the same circumstances without fecundation, 

 though unbroken, turbidity ensues ; so that 

 by the difference of internal appearance the 

 fertile ova soon come to be easily distin- 

 guished from those which have not been fe- 

 cundated.* The solid elements of the yolk- 

 substance appear to be in general of three 

 kinds for some time before the ovum has 

 arrived at maturity : viz., 1st, a quantity of 

 small granules comparable to the granular 

 yolk-substance of the primitive ovum ; 2nd, 

 collections of clearer vesicles and globules 

 interspersed with the first, and in general 

 partly mixed with them and partly situated 

 in a deeper layer ; and, 3rd, the large oil glo- 

 bules. These last are usually somewhat co- 

 loured ; they are comparatively large, and m 

 some fishes are very few in number, and even 

 reduced at last to only one, which is then 

 of proportionally large size. In all fishes, 

 the number of oil globules appears from the 

 observations of Retzius gradually to diminish 

 as the ova approach maturity.f The large 

 oil globules float quite freely in the fluid 

 of the yolk; so that from their greater 

 lightness they always rise towards the side 

 of the ovum which is turned uppermost; 

 but the other elements of the yolk-substance, 

 and especially the small granules of the germ- 

 disc, come in the mature ovarian ovum to 

 occupy one side of the yolk, and, as they 

 form a coherent layer, do not move readily 

 from this place. The smaller granular par- 



* See a paper by Dr. Davy in the Proceed, of 

 Roy. Soc. of Lond. 1852, p. 149. 

 t See Retzius in Muller's Archiv. for 1855, p. 34. 



