77S PROFESSOR W. C. M'INTOSH AND MR E. E. PRINCE ON 



p. 537), about or soon after the time of hatching. Occasionally one or more detached cells 

 may be seen loosely suspended in the auricle, near its external opening, and they swing 

 to and fro with the heart's pulsations. No blood, as such, exists until a later stage, and 

 any fluid included in the lumen of the heart and the pericardium must be non-corpus- 

 culated, and its presence cannot be demonstrated. It may be doubted whether the stray 

 corpuscles above referred to are blood-elements at all, for Lereboullet is almost certainly 

 correct when he says that it is erroneous to assert that the corpuscles which first appear 

 in the heart are detached from its walls: "they are different in character, and too 

 coherent to become detached" (No. 93, p. 585). In our specimens these had the appearance 

 of papillae on the cardiac wall. Ryder regards the periblast as the source of the blood- 

 corpuscles, in accordance with Hoffman (Zool. Anz., 1880, p. 633) ; and in this view 

 their connection with the so-called free nuclei around and beneath the early blastoderm 

 is naturally suggested. Eyder contends (No. 141, p. 537) that the pericardial cavity 

 is really the persisting segmentation- (or more correctly, germinal) cavity, and that the 

 passage of periblastic blood-elements into the heart is thus secured. It must be 

 remembered, however, that the roof of the germinal cavity consists of the subembryonic 

 hypoblast, a layer which stretches beyond the tip of the snout of the young fish, and 

 extends as the under-stratum of the double-layered yolk-sac (ys, PL V. fig. 8). This 

 subcephalic chamber, with its floor of periblast and roof of hypoblast, is never obliterated ; 

 but though its periblastic floor does not bud off cells to form the ventral half of the 

 mesenteric wall, yet its roof (ys) becomes pushed downward (vide PL XII. figs. 1, 3) 

 until it lies below the pericardium (PL XII. fig. 2 ; also see PL VIII. fig. 6), and is 

 separated only by a narrow fissure from the periblast (per) beneath. The germinal 

 cavity diminishes in a less degree laterally, and the latero-pharyngeal spaces into which 

 the embryonic breathing aperture opens from without (see p. 747) are its more visible 

 remnants (ss, PL IV. fig. 21 ; PL XL figs. 6, 7, 8). The floor of the pericardium 

 appears (vide PL IV. fig. 21) to be obliterated anteriorly, but even in this case 

 the delicate hypoblast would seem still to separate the pericardial from the germinal 

 cavity below. The absence of the limiting layer from a certain area may be explained 

 also, not by obliteration, but by a different method of origin, and it is quite possible that 

 the pericardium may be a fold of mesoblast directed forward. Truman, indeed, speaks 

 of such a mode of development in Esox, a membranous fold being reflected from the 

 under part of the head (No. 154, p. 190). 



Meanwhile the vascular canals of the trunk are in course of formation, a small arterial 

 vessel (the dorsal aorta) being hollowed out of the loose trabecular tissue (really the 

 intruding mesoblastic cells above the gut which are broken down) along the under side of 

 the notochord (x, PL VII. figs. 1, 4, 6 ; PL XII. fig. 8), and two venous trunks of large 

 calibre are similarly formed in the lateral connective tissue just external to each head- 

 kidney. In the living larva of Molva vulgaris, on the fourth day, the subnotochordal 

 tissue seems to be traversed by a single large vascular channel (vn) separated by an 

 interval, probably the dorsal aorta (ao), from the chorda (nc). The large channel can 



