DEVELOPMENT AND LIFE-HISTORIES OF TELEOSTEAN FISHES. 691 



successfully fecundated — a series of ova of the species named being fertilised sixteen 

 hours after oviposition ; and in the case of the ova of the herring from the deck of a 

 fishing boat, G. Brook states that forty-eight hours have been allowed to elapse, yet fertil- 

 isation was found to be successful. 



More uncertainty probably exists in the case of pelagic ova, which after expulsion are 

 never quiescent, but may travel over large areas, so that at times their fertilisation must 

 be a matter of chance. The fishes at the spawning season congregate, it is true, in vast 

 numbers, males and females thus herding together ; but ripe females may occasionally 

 shed their ova where it is problematical whether sperms will ever reach them, and. in this 

 way we can account for the quantity of dead eggs of plaice and cod which Hensen found 

 while dredging in the inner bay of Kiel (No. 65, p. 429), though changes in the nature of 

 the water have also to be taken into account. If no spermatozoa reach them within a 

 limited time after extrusion, pelagic eggs lose their glassy transparency, and descending to 

 the bottom assume the white opacity and wrinkled appearance of dead ova. In demersal 

 forms, with a denser capsule, the unhealthy or dying condition is not so readily seen; 

 but opacity of the contents, and especially an increasingly offensive odour, if in masses, 

 are unmistakable indications of loss of vitality.""" 



The relation of the micropyle to effective fertilisation has been already treated of ; 

 and many authors regard its position as of the highest importance. Gerbe, indeed, 

 satisfied himself in the case of the trout that this position is always superior, and 

 he took pains to secure this condition when performing artificial fertilisation (No. 57, 

 p. 330). In the uppermost segment he found after fertilisation that a granular layer is 

 formed by a process of thickening, so that a " nuage vague " condenses as a circular area 

 always in relation to the micropyle. Gerbe would extend the observations he noted in 

 the trout to the ova of Teleosteans in general, and certainly in many demersal forms the 

 blastodisc concentrates in the uppermost segment, and. the micropyle is stated to be 

 uppermost ; yet in pelagic eggs the disc would appear always to be formed at the inferior 

 pole, and in such eggs if the constancy of the position of the micropyle be well founded, 

 it must be no longer uppermost, but on the under side of the egg, and such is affirmed to 

 be the case, though there are difficulties in the way of such an affirmation, and many 

 reasons for holding that the position is not necessarily constant. 



Demersal ova do not show a uniformity in the situation of the micropyle, for in the 

 egg-tubes of Perca it is not uppermost, but directed to one side, so that it opens into 

 the lumen of the cylinder; and Gerbe found that it occupies a like position after 

 fertilisation in Salmo fario, the capsule he says, moving through a quadrant, so that the 

 micropyle is no longer uppermost ; " this change simply alters the respective positions of 

 the cicatricula and the micropyle, and when accomplished the phenomenon to all intents 

 and purposes is ended" (No. 57, p. 331). 



* The ova of Osmerus eperlanus would seem to become opaque very rapidly, for Cunningham notes that the 

 unfertilised eggs sank to the bottom, remained unattached and free, and became opaque-white shortly after expulsion, 

 though at first they were of a translucent yellow (No. 49, p. 293). 



