Development of the Food-Fishes. 445 



globule of a pale salmon-tint ; (2) a delicate cortical film of 

 protoplasm, in which small vesicles and granules occur ; (3) 

 a narrow space, the " breathing-chamber " of Newport*, 

 separating the vitellus from the external capsule, and permit- 

 ting it to revolve freely within the latter ; lastly, the ovum 

 possesses (4) an external protective membrane, the yolk-sac 

 of Ransom f. It is structureless, tough, hyaline, destitute of 

 pores or striations, slightly resilient, and varies in thickness 

 in different species, though always comparatively thin and of 

 great transparency. It is of uniform thickness in the same 

 ovum, being most tenuous in P. limanda, measuring not more 

 than "0001 inch in thickness. It is slightly denser in P. 

 Jlesus, "000127 inch; in Gadus morrhua it measures '00025; 

 but it is considerably thicker in the ovum of T. gurnarduSy 

 being no less than '0005 inch. One aperture pierces the 

 capsule, and its structure is the same in tlie several species, 

 exhibiting an " hour-glass " form, with a crater-like external 

 and a larger internal opening. 



Deposition. 



As tlie ova mature they pass posteriorly, and descending to 

 the genital aperture are readily expelled. Differences in the 

 manner and duration of spawning doubtless obtain, as already 

 stated, in the various species of food-fishes ; but the ovarian 

 walls, assisted by the abdominal parietes, at this time much 

 distended, probably in all cases effect the extrusion of the 

 ova. Very slight pressure upon the abdomen of a well- 

 developed female causes the eggs to issue in a continuous 

 stream, and artificial spawning may be easily perfoi»med. 

 Amongst the Pleuronectida3 cases frequently occur of egg- 

 bound females, in which the contained ova are translucent 

 and mature^ but, from diiiiculty in expulsion, are retained 

 beyond the proper time. Such ova, when artificially extruded 

 and fertilized, may develop in due course, though others 

 under observation did not survive. A lubricating fluid facili- 

 tates expulsion ; but it possesses little adhesive property, and 

 is not hardened by contact with sea-water, unlike demersal 

 or non-pelagic ova, which are often firmly bound together by 

 this means. It is an interesting fact that the undetermined 

 ova studied by Hiickel and E. van Beneden|, though pelagic, 

 adhered together in masses, and that the ova of Lophius 

 piscatorius float in masses of mucus. Upon expulsion the 

 buoyancy of these pelagic ova is at once apparent. Though 

 unfertilized they rise to the surface, as Prof. M'Intosh and 



* Phil. Trans, vol. cxli. 1851. t Ibid. vol. clvii. p. 433. 



X Quart. Journ. Micr, Sci. vol. xviii. 1878, p. 42. 



