700 PROFESSOR W. CO. M‘INTOSH AND MR E. E. PRINCE ON 
cleavage-process. The view that it is simply the cortical protoplasm, and not a definite 
membrane (vide No. 122, p. 445), is supported by certain facts which Ransom mentions, for 
he speaks of the inner face of the yolk-sac as ill-defined and closely connected with the 
formative yolk (No. 127, p. 433), and that on rupture the shreds change their form 
(Ihid., p. 478) and are frequently drawn out into thread-like prolongations (p. 468) ; 
while he further describes it as continuous with the blastoderm (p. 467), and admits 
that, as it ultimately shares in the cleavage process, it ‘‘ may to that extent be considered 
a part of the formative yolk” (p. 433) or germinal protoplasm. ‘The presence of a 
like membrane investing the germ has been maintained by ScHENK in the ovum. of 
Elasmobranchs (No. 142), but other observers, including Leypiec and Batrour, have 
denied its existence. The yolk-sac described in the hardly mature ovum of Rana by 
Cramer (No. 45, p. 33) as a distinct membrane before cleavage begins, is merely the 
more consistent superficies of the yolk-ball, and not a separable structure. The fact 
seems to be that what Ransom regards as a distinct membrane is the cuticular stratum 
of the protoplasmic cortex, and is therefore less of the nature of a sac than that of an 
external layer, slightly more consistent than the protoplasm underneath. Ransom 
admits that in a sense it may be so regarded (No. 127, p. 433); and it is adherent to 
the blastodise, over the outer surface of which it passes, and probably constitutes the 
clear matrix, as distinct from the granules of the dise. It forms folds at the margin of 
the clefts during segmentation, “ reminding one,” he says, “of the ‘ Faltenkranz,— 
described by RetcHErT and by Scautrze in the frog’s egg,”—these folds being in fact 
the familiar corrugations produced by the cleavage and separation of the blastomeres. 
Sections through the dise at this time show no investing membrane, though it is true 
that the cortex takes a slightly deeper stain than the underlying matrix of the 
blastomeres, but the one insensibly passes into the other. Barour also found, in the 
ova of Elasmobranchs, that the surface was very susceptible to stains, and that the sides 
of the furrows took a deep colour ; but such appearances did not suffice, in his view, to 
demonstrate a separate membrane, so that in Teleosteans, also, we must, with LEREROULLET, 
affirm “labsence de membrane propre” (No. 95, p. 13) outside the blastoderm. That 
Ransom’s layer is simply the cortical protoplasm is shown by the fact that on rupturing 
it no coherent layer beneath held in the contents, but the food-yolk immediately flowed 
out (No. 127, p. 465). Ransom himself also speaks of the formative yolk as a layer invest- 
ing the yolk-ball. We cannot, therefore, recognise an inner yolk-sac as such, for the 
somewhat viscid and coherent layer, which alone appears to envelop the yolk, would 
behave precisely as Ransom’s yolk-sac did, when in contact on its inner side with the 
semi-fluid yolk, and on its outer side with the watery perivitelline fluid. The whole 
of this cortical protoplasm, however, does not enter the blastodise and undergo seg- 
mentation ; a considerable part never reaches the animal pole, but permanently clothes 
the yolk-globe, and part of it may temporarily form a supplementary disc at the 
vegetal pole, as Kuprrer saw in Clupea (No. 87, p. 185); while a portion remains as a 
sub-blastodermic stratum, and becomes thickened as a peripheral wall, the nuclear zone, 
