NORTH SEA INVESTIGATIONS. 367 



That the ova of an individual female are not shed all at once, but 

 in successive crops, is well known (cf, Fulton, Comparative Fecun- 

 dity of Sea Fishes, Rep. S. F. B., 1891, p. 245), and I think that 

 the different degrees in which the retention of ripe ova obtains in 

 different species must be explained by the difference in the size of 

 and interval between the several crops. In the plaice, and in some 

 other species, a greater number of ova, compared to the capacity of 

 the ovary, ripen together, and the successive crops appear to over- 

 lap each other to some extent, so that there never seems to be a 

 time when some ova are not ready for extrusion. Consequently 

 the muscular efforts for extrusion have the co-operation of the con- 

 stant and rapid increase of the successional crops, so long as any 

 such remain ; but when it comes to the turn of the last crop the 

 elasticity of the walls of the ovary and the muscular power of the 

 parent are so insufficient that a comparatively large number fail to 

 make their escape. 



In the lemon sole, on the contrary, I have evidence that the crops 

 are individually small (compared to the capacity of the ovary), and 

 separated from each other by intervals which, though imperceptible 

 in the spawning period of the species, must be well marked in in- 

 dividual parents. The fish must therefore possess resources, mus- 

 cular or otherwise, which render it practically independent of the 

 assistance afforded by the successional crops in extruding the ripe 

 ova (the difficulty being lessened by the comparatively small number 

 of the latter which are simultaneously ready), and thus does not 

 encounter any unusual obstacle in disposing of the last crop. 



Before dealing with the topographical features of the ovary in 

 its different conditions we must briefly advert to the modifications 

 exhibited in the disposition of the viscera in the various species. 



As is well known, the ripe ovaries in any flat-fish are disposed on 

 either side of the hgemal spines, and extend from a point in the 

 neighbourhood of the bony buttress which forms the centre of the 

 hind wall of the peritoneal cavity to a greater or less distance from 

 the commencement of the caudal peduncle. A single ovary duct, 

 formed by the coalescence of the duct from either ovary, follows the 

 course of the buttress in its anterior curve to the genital orifice. 



But while all have this much in common in the ripe condition, 

 the British representatives of the family fall into two groups, based 

 upon the arrangement of the alimentary canal and the shape of the 

 reproductive organ : 



(a) including the halibut, long rough dab, turbot, brill, megrim 

 (scald-fish and topknots ?), plaice, flounder, and common dab. 



In these no portion of the alimentary canal extends to any con- 

 siderable distance beyond the first ha3mal spine. The ovaries are 



NEW SERIES. — VOL. II, NO. IV. 28 



