Yolh in the Eggs of Osseous Fishes. 5 



bulk of the yolk in the earlj ovum (PI. II. fig. l,g) and in a 

 later stage when the embryo is fairly advanced, as in PL II. 

 fig. 3,g, and the very slight diminution that does occur (vide 

 PI. II. fig. 4, 3/) can be accounted for by the continued sepa- 

 ration of the interfused protoplasm. The large size of the 

 yolk-mass, in the emerged embryos of pelagic and demersal 

 forms alike, indicates that any very active process of digestion 

 is doubtful. That the globular ball of yolk is not an integral 

 part of the germ or embryo is sufficiently shown by the ease 

 with which it can be removed from its periblastic and embry- 

 onic envelopes in hardened specimens. The yolk seems to be 

 chiefly utilized during the early stages of the active liberated 

 embryo, diminishing greatly during the first fortnight after 

 hatching (compare figs. 4 and 6,?/, PI, II.), and in those 

 species which develop a vitelline circulation the rapid 

 removal of the yolk-granules can be readily understood. In 

 pelagic forms, without such vascular provision, the yolk is 

 less rapidly used up ; and, doubtless, in these the coeliac and 

 hepatic blood-vessels, being in close proximity to the yolk- 

 surface, efiect the absorption. 



All this evidences the accessory nature of the yolk in 

 Teleosteans. It is an appendage — a c^nogenetic addition or 

 adaptation, as Hackel regarded it — not directly contributing 

 to the building up of the tissues, but mainly serving to fur- 

 nish pabulum to the delicate and rudimentary embryo on 

 emerging from the egg It is not more essentially connected 

 with the development of the germ than the egg-envelope *. 

 In hardened preparations it shows a granular structure, and 

 when physically manipulated often has the texture of dense 

 cork ; and in the young salmon, as Professor M'Intosh long ago 

 described, the yolk becomes less fluid, and by-and-by springs 

 from the touch of a glass rod like a rounded and smooth bit 

 of cartilage on simply transferring the embryo from fresh to 

 salt water "j". In the living egg it is a clear albuminoid 

 matrix of the consistency of syrup, readily issuing from a 

 puncture in the yolk-sac (PI. II. fig. 5, y), and containing 

 minute vesicles and refrangible particles, with the addi- 

 tion, in certain species, of large oleaginous spheres. The 

 presence of these spheres in the yolk adds strength to the 

 view that it is a nutritive appendix, for, as shown in a pre- 



* Vide Quart. Joiu-n. Microsc. Sci. toI. xvi. 1876. — Note on p. 06, wliere 

 Prof. Ray Lankester distinguishes the added food-material and egg-enve- 

 lopes as " matrificial " and not " ovificial " elements, like the protoplasm 

 of the egg-cell proper. 



t Quart. Journ. Microsc. Sci. vol. viii. 1868, p. 163. 



