782 PROFESSOR W. C. M‘INTOSH AND MR E, E. PRINCE ON 
figs. 1, 2, and 3, and Pl. XVII. fig. 2). This does not take place, however, to any very 
appreciable extent while the embryo is within the ovum, whereas the reduction is 
very marked in Perca (No. 93, p. 610), Cyclopterus, and similar species. After the 
embryo emerges in pelagic forms, and before any circulation of a corpusculated hemal 
fluid exists, the yolk, which is very large and prominent in the newly hatched fish, 
becomes speedily diminished. A process of absorption must be actively going on in 
these forms (e.g., cod), and the presence of a transparent plasma hathing the tissues, and 
filling the pulsating heart and lacunz of the trunk, is suggested. 
The origin of the blood-corpuscles is an interesting point; but there is little 
unanimity amongst observers on this matter respecting Teleosteans, and appearances seem 
to support more than one suggested mode of origin. Ryprr, with Horrman and others, 
as we have already said, holds “that the blood-cells are budded off directly” from the 
periblast, the nuclei of which layer by division give rise to groups of granules, the form- 
elements of the blood (No. 141, p. 543). C. Voer in 1842 distinguished a “ couche 
hematogéne” (No. 155), as did also RatHKe and Von Bakr, their third or vascular layer 
of the blastoderm being, however, derived from the “lower layer” or hypoblast-cells ; 
and Van Bamsekg, while admitting that the periblast or “intermediary layer” has not 
been proved to be this ‘ vascular layer,” appears to consider their homology very probable 
(No. 20a, p. 9). Genscu’s researches support this view, the corpuscles arising from the 
layer surrounding the yolk—‘ Kuprrer’s secondary entoderm.” In opposition to 
Kurrrer’s affirmation that the outer mesodermal yolk-sac gives origin to the corpuscles, 
Genscu found that in Hsow and Zoarces viviparus no mesoblast was present in the region 
where they arose, the two-layered epiblast lying upon the granular periblast in which cells 
were imbedded. These cells give out pseudopodial processes, which are constricted off to 
form corpuscles, and these by subdivision produce blood-islands (vide No. 56). In Salmo, 
Alosa (No. 141, p. 537), Gastrosteus (No. 122, p. 494), and other forms, the phenomenon 
described by Grenscu has been observed, yet it is not conclusive that the primary 
corpuscles are derived from the “ Dottersack.” That the periblast contributes to the 
nutrient hemal fluid of the embryo there can be no question, but the point of chief 
moment is, whence are the primary corpuscles derived? As LerEBouLLEer long ago 
pointed out, the heart beats for some time before corpuscles appear in its lumen ; and he 
added that the heemal trunks too are formed, as in the gurnard, before the corpuscles 
(No. 93, p. 577). Werncxepacn, however, holds that in the process of formation the 
blood-vessels give origin to the corpuscles, so that both originate contemporaneously. 
This observer concludes that the blood-corpuscles appear to him to arise in a solid mass 
of tissue in the region where the vena vertebralis is afterwards situated, the cells 
constituting this mass being carried away by a hemal plasma, and acquire the colour 
and character of blood-corpuseles subsequently (No. 157). The polyhedral cells which 
Wenckepacu shows filling up the lumen of the subnotochordal vein (vide No. 157, 
pl. viii. figs. 2, 3, &e.) are also found, in section, to fill up the aortic trunk, and there 
is no reason why the derivation of these blood-cells should not be extended to all the 
