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 haemal 

 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 bathing the tissues, and 

 filling the pulsating heart and lacunae 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. Ryder, with Hoffman 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. Vogt in 1842 distinguished a " couch e 

 haematogene" (No. 155), as did also Rathke and Von Baer, their third or vascular layer 

 of the blastoderm being, however, derived from the "lower layer" or hypoblast-cells ; 

 and Van Bambeke, 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). Gensch's researches support this view, the corpuscles arising from the 

 layer surrounding the yolk — " Kupffer's secondary entoderm." In opposition to 

 Kupffer's affirmation that the outer mesodermal yolk-sac gives origin to the corpuscles, 

 Gensch found that in Esox 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 Gensch has been observed, yet it is not conclusive that the primary 

 corpuscles are derived from the " Dottersack." That the periblast contributes to the 

 nutrient haemal fluid of the embryo there can be no question, but the point of chief 

 moment is, whence are the primary corpuscles derived ? As Lereboullet long ago 

 pointed out, the heart beats for some time before corpuscles appear in its lumen ; and he 

 added that the haemal trunks too are formed, as in the gurnard, before the corpuscles 

 (No. 93, p. 577). Wenckebach, 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 vcrtebralis is afterwards situated, the cells 

 constituting this mass being carried away by a haemal plasma, and acquire the colour 

 and character of blood-corpuscles subsequently (No. 157). The polyhedral cells which 

 Wenckebach shows filling up the lumen of the subnotochordal vein (vide No. 157, 

 pi. viii. figs. 2, 3, &c.) 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 



