48 REPRODUCTION AND DEVELOPMENT OF TBLEOSTEAN FISHES 



able to identify them with any certainty. The ova most difficult to 

 identify are those which have a homogeneous yolk with a single oil- 

 globule, because there are so very many species which have ova of 

 this character differing only, in the early stages, in size, and some- 

 times not even in that respect. Further and more minute study of 

 the larvae will probably enable us to distinguish the species to 

 which they belong, but before hatching this is almost impossible. 

 It has been seen ali-eady in this paper that this type of ovum is 

 common to the families Scombridae (mackerel), Carangidae (cuckoo), 

 many G-adidae (hake, rockling), many Pleuronectidae (Arnoglossus, 

 Rhombus). And Raffaele describes ova of the same kind from the 

 bass, Labrax lupus (Percidse), and from some of the sea- breams 

 Pagellus erythrinus, &c. (Sparidae) ; some of the wrasses (Labridae), 

 namely, Coris and Julis, have also similar ova. 



It is evident that a great deal still remains to be done before an 

 adequate knowledge of the development and growth of the Plymouth 

 fishes is obtained. But the researches I have described in this 

 paper have been, and future work will be, greatly facilitated by 

 Raffaele's admirable memoir so often quoted in the preceding 

 pages. The extent to which the Italian naturalist's results apply 

 to the fish-fauna of Plymouth, shows how closely connected fauna- 

 logically are the south coast of Britain and the Mediterranean. 



A Hypothesis conceening Oil-Globules in Pelagic Teleostean Ova. 



In considering the question why some pelagic ova have separate oil- 

 globules in their yolk while others have none at all, I have noticed 

 a connection between the presence of these separate masses of fatty 

 matter and the normal quantity of oil in the body of the parent 

 fish. Whenever the adult has a large quantity of oil in its tissues 

 the ova possess one or more oil-globules in the yolk. At least this 

 seems obvious in some cases. In both the herring and the pilchard 

 the yolk is completely vesicular ; this is a character common to all 

 species of the genus Glupea ; but the pilchard ovum has a large oil- 

 globule, the herring ovum has none ; and it is certain that the 

 adult pilchard possesses more oil in its tissues than the herring. 

 The mackerel also is a rich oily fish, and its ovum has an oil- 

 globule, while the species of Pleuronectes and Gadus have com- 

 paratively dry flesh, and their ova have no oil-globules. The sole 

 is richer in oil than the plaice, and the former has oil-globules 

 while the latter is destitute of them. 



There are two ways of regarding this fact if it be one. We may 

 suppose that the excess of oil runs over as it were into the ova. 



