DEVELOPMENT AND LIFE-HISTORIES OF TELEOSTEAN FISHES. 699 



the protoplasm interfused with the yolk also collects at the surface, though it is not 

 visibly separated by a line of demarcation, and can only be recognised by its texture and 

 property of readily staining. Ere long it completely separates from the granular deuto- 

 plasm, and forms a superficial blastodermic layer enveloping the yolk.* In the same 

 manner a protoplasmic cortex, like the periblastula just mentioned, forms an equal layer 

 over the yolk in fishes' eggs, but is not at first sharply defined, though later it is so. 

 Balfour observes that in Elasmobranchs the disc is merely a part of the ovum in which 

 the protoplasm is more concentrated, and the yolk-spherules smaller than elsewhere. 



In the ova of the haddock on the second day the blastodisc shows small " oil-globules " 

 amongst the protoplasm between the spheres, and the disc presents a pale salmon-tint by 

 transmitted light. Usually it appears to consist of homogeneous protoplasm, with numerous 

 small spheres of oil or indifferent fluid and scattered granules. In Clwpea harengus no 

 cortical layer is present before segmentation, according to Kupffer (No. 87, p. 179), nor 

 is a blastodisc preformed, this latter feature being shown also by Gadoid and other 

 pelagic ova, though in these eggs a cortical layer is well defined before fertilisation. 

 Notwithstanding that the cortex seems thus sharply marked off from the yolk, there is 

 good reason to believe that the centrifugal movement of the deeper interfused protoplasm 

 does not cease when the layer is formed, and Klein refers to this process as the feeding 

 of the cortex upon the yolk for purposes of growth (No. 79). Balfour also speaks of 

 certain nutritive elements of the yolk as being converted into protoplasm (No. 11, note 

 at foot of p. 679), and Kupffer (No. 88, p. 214) and Rieneck (No. 137) have adopted a 

 similar view, as also more recently has G. Brook (No. 30). 



No nuclei can be detected in the cortex ; but clear structureless spheres occur in 

 small groups, or singly over its surface, and these coalesce later, and form larger spheres, 

 which are found at the base of the blastodisc during segmentation. Ryder has 

 determined their composition to be that merely of an indifferent fluid (No. 141, p. 467). 

 Outside this cortical protoplasm Ransom distinguishes a delicate homogeneous layer, his 

 " inner yolk sac," which is not possessed by the more immature eggs. In "the smallest 

 intra-ovarian ova " examined in saliva, he says " the yolk is granular and irregular, not 

 smoothly defined as it would be were an inner sac present " (No. 127, p. 442) ; and in ova 

 two-thirds their full size, also, he failed to perceive it. When intact it seems able to 

 resist osmotic currents in Salmo solar, and it varies in bulk, being unusually thick in the 

 ruffe (Acerina) (Ibid., p. 453). 



Such an inner-sac would appear to be absent in Gadoid and similar pelagic ova, 

 and indeed in the forms studied by Ransom the precise nature of the so-called inner-sac 

 is a subject for further investigation. He regards it as a membrane, as performing 

 contractile movements, and as folded in along the lines of blastodermic cleavage 

 (No. 127, p. 479). 



It is difficult, however, to conceive a structure, meriting the name membrane, envelop- 

 ing yolk and germinal disc so closely as to be almost inseparable, and involved in the 



* Vide Reichenbach, "Die Embiyonalanlage und erste Entwickelung des Flusskrebses," Zeit. f. w. Z., xxix., 1877. 



