DEVELOPMENT AND LIFE-HISTORIES OF TELEOSTEAN FISHES. 703 
(1) the blastodise, or true segmenting mass; (2) a granular layer, or subgerminal dise not 
segmenting, and probably nutritive, and interposed between No. 1 and the vitellus. 
V. Tue BLastopERM. 
Within one or two hours after the entrance of the spermatozoa, the thickened cap 
of protoplasm, either preformed as a discus proligerus, or segregated as a blastodise 
proper, undergoes segmentation. The blastodise is readily distinguished with the 
naked eye in the more transparent ova as a spot of lighter colour than the yolk 
on which it is placed; while under a lower power it is seen protruding as a discoid 
prominence at either the upper or the lower pole, according to the particular form. 
In certain Salmonide, for instance, the germ always floats uppermost, as it also does in 
the sterlet, according to SALENSKY, and in the trout; this being due, according to Ransom, 
to the oil attached to the disc, which compels it to float in the upper segment (No. 127, 
p- 450).* Ina number of pelagic ova, possibly in all, the dise lies underneath the yolk, 
the animal pole being inferior ; but whether superior or inferior, the position is constant 
for the species, and there is no actual reversal, such as occurs in Cephalopods, where the 
germ and the yolk-pole exchange places at a certain stage. As the vitelline mass revolves 
freely in the perivitelline fluid, the germ may often be brought to the upper side by 
agitation in the water; but it usually seeks the lower pole at once, and remains there 
when the egg is at rest. 
Baxrour views the disc merely as a part of the ovum, which is characterised by the 
presence of more protoplasm than the rest of the vitellus (No. 10, p. 106); but while 
this is so in the Elasmobranch and Amphibian ovum, in the Teleostei the germ is so well 
marked and distinct, and, with the exception of some colourless vesicles and a few granules, 
so destitute of yolk-matter (apparently consisting of pure protoplasm) that the yolk 
becomes rather an appendix than an essential part of the germ. 
The same author supposed that the Teleostean yolk at some later stage must be almost 
entirely deprived of the protoplasm so abundantly interfused during the early stages, 
and this undoubtedly is so, the yolk-matrix before it wholly disappears increasing ‘in 
density and coherency.t That the disc owes its origin to fecundation in all Teleosteans, 
we have seen to be an error; and the view of Cosrr, which LEREBOULLET adopts (No. 93, 
p. 33), is not more tenable—viz., that the dise is derived solely from the divided and 
scattered germinal vesicle—for, in some species, the discus proligerus is formed and this 
vesicle is seated in its midst. As the segregation of the disc proceeds, and its mass 
increases, its colour likewise becomes deeper ; and Ransom believes that it undergoes a 
physical change, “ being more solid” than in its earlier condition. 
The dise then is the essential part of the ovum, and the yolk is merely supplementary, 
* His figures the germ disc of Zsox as uppermost (No. 67, Taf. i. fig. 13); but LEREBOULLET says, “ Sa position est 
oblique ou, si l’on vent inclinée & l’équator” (No. 93, p. 481). 
+ In a form like Anarrhichas the embryo remains long (several months) within the ovum, and when treated with 
alcohol the yolk becomes extremely hard, and apparently consists of a purely albuminoid matrix. This likewise is the 
case with the ovum of Salmo salar. Sea water also hardens the yolk of the latter species (vide No. 104a, p. 153). 
