OVULATION IN OSSEOUS FISHES AND BATBACIIIANS. 593 



distinguished : those about to be discharged are large and dark- 

 coloured, those intended for the next season are also of uniform 

 size, but smaller, and partially coloured, and the rest are much 

 smaller, colourless, and varying in minuteness. In Plagiostomes 

 the ova are fewer in number than in the 'roe-fish.' From four 

 to fourteen ova, for example, may be developed at one season in 

 the Torpedo (T. marmorata?),^ whilst in the Herring 

 2.5,000 ova, in the Linn])-fish lo5,000 ova, in the 

 Ilolibnt, 3,500,000 ova, have been estimated to fill 

 the enlarged ovarian sacs. In a Lmnp-fish, the 

 total weight of which was 9 lbs. 8 ounces, or 66,500 

 grains, the ovaries weiglied 3 lbs. 3 ounces, or 22,300 

 grains : thus they were to the body as 1 to 3. Each osseous FiSii 

 ovum weighed one-seventh of a grain.^ uuignined. 



In all Fishes the ova are formed in chambers of the ovary, 

 called ' ovisacs.'' In Osseous Fishes the ovisac consists of a 

 delicate membranous hollow sphere, fig. 415, a, lined by 

 epithelial nucleate cells, and surrounded by a tliin layer of the 

 proper tissue, or ' stroma,' b, of the ovary ; which, as it protrudes 

 with the growth of the ovum into the ovarian cavity, carries 

 before it a covering of the delicate mucous membrane lining 

 that cavity. This tunic is not present in Cyclostomes and Pla- 

 giostomes. The first-formed and essential part of the ovum 

 is the germ-cell, or ' germinal vesicle,' c, which, in Osseous 

 Fishes, shows several nuclei, macula3, or ' germ-spots,' d, but in 

 Plagiostomes only a single nucleus. Around the germ-cell there 

 accumulates a collection of minute yolk-corpuscles and albuminous 

 granules, e, with oil-like globules, /", and in some species (Carp) 

 flat angular corpuscles are added : all are suspended in a clear 

 gelatinous yolk-fluid, and are ultimately circimiscrilicd by a delicate 

 yolk-membrane, g, devoid of visible structure. The increase of 

 the ova is due chiefly to the accumulation of the yolk, and its 

 colour to that which the oil-globules acquire as the ova approach 

 matiu'ity. Finally is formed the external tunic, or ' ectosac.'' At 



' CXSXII. 



^ cccviii. p. 49. The periodical, but r.apid and enormous increase of the hard and 

 soft roes in osseous fishes admits of no rigid cinctures, no unyielding bony hoops 

 around the abdominal eaTity, such as would have resulted from a conversion of the 

 ])Ieurapophyses, by their junction with liaimapophyses and a sternum, into ' true ribs.' 

 We sec, therefore, in the fecundity of fishes — in this compensation for their limited 

 intelligence and numerous foes— the physiological condition of their free or 'floating' 

 ribs. ' xs. iv. 1838, p. 131. 



■* As the homology of this tunic is not clearly determinable either with the vitelline 

 membrane of the ovum of the Bird, or with the chorion of that of the Mammal, it \3 

 indicated by the above term in the description of the ovum of the Osseous Fish. 

 VOL. I. Q Q 



