294 LECTURE XII. 



cessively in the Plagiostomes. They are much fewer in number in 

 the latter : from four to fourteen ova, for example, are developed at 

 each breeding season in the Torpedo marmorata* In all fishes the 

 ova are formed in chambers of the ' stroma,' called ' ovisacs.' In 

 osseous fishes the ovisac f consists of a delicate membranous hollow 

 sphere — the ' ovarian vesicle' (Jig. 77. a) \, suiTounded 

 by a thin layer of the proper tissue or ' stroma' {b) of the 

 ovary, which, as it protrudes into the ovarian cavity, 

 carries before it a covering of the delicate vascular mu- 

 ^ cous membrane. This tunic is not present in Cyclo- 



Ovarian Ovum : stomcs or Plagiostomes. The ovum consists of the pri- 

 mlg^ifi'ed"'' mordial or germinal vesicle, ' germ-cell' (c), which, in 

 osseous fishes, shows several nuclei or 'germ-spots' {d\ 

 but in Plagiostomes only a single nucleus. Around the germ-cell 

 there accumulates a collection of minute yolk-granules (e) with oil-like 

 globules if), the whole contained in a delicate yolk-membrane (g). 

 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 ap- 

 proach maturity. At this period the ova in osseous fishes escape into 

 the cavity of the ovarium ; and to their then outer covering, the yolk 

 membrane, is added a second tunic called ' chorion.' The ovisac re- 

 mains behind and coalesces with the stroma, to form, according to 

 Barxy § , a " vesicle analogous to the Graafiftan vesicle of Mammals : " 

 but the evacuated ovisacs collapse and speedily disappear in the 

 shrunken ovarium, after the discharge of the ova, in Dermopteri and 

 osseous fishes : they are longer recognisable in the Skate. 



The periodical, but rapid and enormous increase of the hard and 

 soft roes in osseous fishes, admits of no rigid cinctures, no un- 

 yielding bony hoops around the abdominal cavity, such as would 

 have resulted from a conversion of the pleurapophyses, by their 

 junction with hasraapophyses and a sternum, into 'true ribs.' We 

 see, therefore, in the fecundity of fishes, — in this compensation for 

 their limited intelligence and numerous foes, — the physiological con- 

 dition of their free or ' floating' ribs. 



Fecundation. — Certain changes and peculiar phenomena attend the 

 increase of size of the soft and hard roes during these primary ^^ro- 

 cesses of generation. The colours of the fishes become more marked 

 and brilliant ; the different sexes are often distinguished by peculiar 

 tints ; as the male Stickleback by his bright red throat, for example. 

 The claspers in the male Plagiostomes then acquire their full develop- 

 ment and force ; the basal glands in those of the Rays enlarge. As 



* Lxxxi. f XX. iv. 1838, p. 131. 



I ' Ovisac ' of Barry, evil. 1st series. § cvii. 1st series, p. 314. 



