700 PROFESSOR W. C. 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 

 (Ibid., 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 Leydig and Balfour, 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 blastodisc, over the outer surface of which it passes, and probably constitutes the 

 clear matrix, as distinct from the granules of the disc. It forms folds at the margin of 

 the clefts during segmentation, " reminding one," he says, " of the ' Faltenkranz,' — 

 described by Reichert and by Schultze 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 disc 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. Balfour 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 Lereboullet, 

 affirm "l'absence 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 blastodisc 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 Kupffer 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, 



