778 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 swmg 
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, Ryprer regards the periblast as the source of the blood- 
corpuscles, in accordance with Horrman (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. Ryprer 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 mto 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. XI. 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 sow, 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 
