366 NORTH SEA INVESTIGATIONS. 



(i) Size and appearance of ovary, 

 (ii) Topographical relationships of ovary. 



(iii) Size and condition of living ova. 



(iv) Presence of ripe ova which have failed to be extruded at 

 the time of spawning. 



Characters (i) and (ii) are essentially dependent on each other, 

 and their interpretation varies with the species in a manner which I 

 shall attempt to indicate below. 



In the next character (iii) we have, I suppose, the clue to the 

 difficulty in distinguishing immature and spent fish, since one 

 may find, in the ovaries of a fish which from its large size might 

 be supposed to have spawned at some previous time, only such 

 " inactive " ova as I have described above. I have, however, 

 never met with such a condition, during the spawning season, in 

 any fish which might not, from its contiguity in size to a series of 

 obviously immature examples, be reasonably supposed to illustrate 

 the variation in the assumption of the mature state. 



It is still too early in the season to ascertain whether a period of 

 inactivity is a regular feature in the rhythm of ovarian development 

 in all Pleuronectids, since some are still spawning, and others have 

 only recently ceased to do so. 



I have met with no such condition in the plaice, which is the 

 earliest spawner of all (with the doubtful exception of the halibut, 

 about which little is known), though I have made somewhat careful 

 inquiries into the ovarian condition of that species since the end of 

 the spawning season. Dabs, however, which presumably spawned 

 about April, noW^ exhibit a germinal epithelium entirely destitute of 

 active ova. 



It need hardly be said that the last character (iv) is an infallible 

 proof of previous spawning when it is present. 



In all species which I have studied a certain number of ripe ova 

 fail to make their escape, and remain to decompose either in the 

 lumen of the ovary itself or in the ovary duct. But their number, 

 and the period during which their presence can be ascertained, 

 appear to vary with the individual and also with the species. Thus 

 I believe that they are most numerous and perceptible for the long- 

 est period in the plaice, perhaps because this species has the largest 

 ova; and I have no doubt the lemon sole is the form in which this 

 character gives us the least assistance. I have received a plaice 

 with ovaries enormously distended with ripe, yet dead and slightly 

 decomposed ova, and though outside pressure caused these to be 

 extruded, it was evident that their deposition was beyond the power 

 of the parent. 



* In September. 



