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. 
Batrour observes that in Elasmobranchs the dise 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 blastodise shows small “ oil-globules ” 
amongst the protoplasm between the spheres, and the dise 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 Clupea harengus no 
cortical layer is present before segmentation, according to Kuprrer (No. 87, p. 179), nor 
is a blastodise 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 Kier refers to this process as the feeding 
of the cortex upon the yolk for purposes of growth (No. 79). Batrour also speaks of 
certain nutritive elements of the yolk as being converted into protoplasm (No. 11, note 
at foot of p. 679), and Kuprrer (No. 88, p. 214) and Rrengck (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. RypEr 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 salar, and it varies in bulk, being unusually thick in the 
ruffe (Acerina) (Ihid., 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 dise so closely as to be almost inseparable, and involved in the 
* Vide Reronennacn, “ Die Embryonalanlage und erste Entwickelung des Flusskrebses,” Zeit, 7. w. Z., xxix., 1877. 
