OOOUREING IN THE NEIGHBOURHOOD OP PLYMOUTH. 39 



"with all reasonable cei^tainty to tliis species. The history of our 

 knowledge on this subject is of much interest, and is an excellent 

 example of the difficulty which may unexpectedly occur in the 

 attempt to solve an apparently simple problem. 



The question of the ova of the pilchard is closely connected with 

 that of floating Clupeoid ova in general, and I will therefore give a 

 brief summary of the history of this larger question. Some years 

 ago the only species of Clupea whose eggs were known with cer- 

 tainty was the herring, Clupea harengus. The eggs of the herring 

 are heavy and adhesive, and when expelled from the body of the 

 fish they stick fast to anything they happen to fall upon. A detailed 

 account of their structure and development was published by 

 Prof. Kupffer in 1878 in the Jahresbericht der Commission zur 

 Untersuchung der deutschen Meere for 1874-76. But before that 

 herring spawn had been dredged from the sea-bottom near the Isle 

 of May in the Firth of Forth. I myself studied the ova in the 

 years 1883 and 1884, and published a paper on them in 1885.* In 

 1882 Alexander Agassiz describedf a pelagic ovum (i. e. a floating 

 one) which had a yolk entirely divided into small segments, and 

 which produced a larva 5 mm. long, having a great resemblance to 

 a larval herring. Agassiz at first thought these eggs must belong 

 to some Clupeoid, but afterwards identified the larva as the young 

 of Osvienis mordax, Gill. But the American Osmerus mordax is the 

 same as the British Oamerus eperlanus, and this species was dis- 

 covered by myself J to have adhesive ova with a peculiar method of 

 adhesion, and to spawn in almost fresh water in the upper parts of 

 estuaries. V. Hensen§ in 1883 described a similar ovum, which he 

 took in the tow-net in the Baltic near Kiel. This ovum, like that 

 of Agassiz, had a segmented yolk and no oil-globules ; its diameter 

 was 1*24 mm., and the larva which hatched from it resembled a 

 herring larva and had a length of 8*7 mm. In 1886|| I described a 

 very similar ovum taken by the tow-net in the Firth of Forth ; this 

 likewise had a segmented yolk, and produced a herring-like larva. 

 The dimensions of the ovum were "94 by '97 mm. diameter, and the 

 larva was 3"63 mm. long. I concluded that this ovum came from 

 the same species as Hensen's. The latter infoi'med me by letter 

 that he believed the ovum to belong to the sprat, Clupea spraitus, 

 but I did not succeed in obtaining fertilized ova of the sprat for 



* On the Significance of Kupjfer's Vesicle, Sfc, Quart. Journ. Micr. Sci., 1885. 

 + Young Stages of Osseous Fishes, part iii, Proc. Amer. Acad. Arts aud Sci., vol. 

 xvii. 



X Proc. Zool. Soc. Lond., 1886. 



§ Vierter Bericht der Comm. Unters. deutschen Meere, II Abtlieilung, Berlin, 1883. 



II Trans. Roy. Soc. Edinb. for Session 1885-86. 



