Cuap. II. FOLDINGS OF THE EMBRYONIC DISC. 559 
22, fie. 9, d, d’) is folded inwardly at several points, a peculiarity which reminds 
one of the faleate process in the eye of certain Birds, and the flabellum in Fishes. 
The two deep depressions at the end of the head, noticed in a former page, (p. 
555, Pl. 24, fig. 12, v,) are here (Pl. 18a, fig. 9, ») very much broadened, but 
at the same time the edges are curved inwardly towards each other. 
The heart (Pl 18a, fig. 8, and 9, /’, /#*) is separated into two very distinct por- 
tions, the auricle (Pl. 18a, fig. 6, 4°) and the ventricle, (4*,) which are joined by a 
narrow tubular isthmus. Although the heart is divided into three chambers, the 
course of the blood is not at all diverted from the channel in which it ran at 
the beginning. In one of our figures, the heart (Pl. 18a, fig. 10, A, 7’) is so 
displayed that its threefold division may be readily seen, and the course of the 
blood easily understood. From the receiving chamber, the right auricle, (/°,) the 
blood passes directly into the left «auricle, (the middle chamber in the figure,) 
and from that into the ventricle, (/*,) and thence, through the bulbus arteriosus, 
(/', fig. 6, 7/’,) into the dorsal artery, etc. The vessels which ramify around the 
brain are becoming very numerous, (Pl. 14, fig. 2a; Pl. 18a, fig. 7, 7°,) especially 
(Pl. 18a, fig. 7, 7) in the neighborhood of the medulla oblongata. The dorsal 
artery: (Pl Se, fies, By 77nd. 8a, 775) Pl... 8a, dig. 7, 93): fier Ta,.97 fig!..8,. 77) 
runs to the tip of the much elongated tail. The omphalo-meseraic artery (PI. 
18a, fig. 8, 7*, fig 9, 7*) is much elongated, and projects as a single vessel, far 
beyond the lower surface of the body, to where the intestine (n', x’) communicates 
with the yolk, and then ramifies, as usual, in the vascular area. The abdominal 
veins (Pl. 18a, fig. 7, 7, fig. 7a, 7, fig. 8, 2?) appear to be in very intimate 
connection with the Wolffian bodies (fig. 7, 9, 7a, g, 8, 7). The allantoidian artery 
(Pl. 18a, fig. 7a, 7°, fig. 8, 7°, fig. 9, 7°) remains single, from its point of origin 
(Pl. 18a, fig. 8, y°) to far beyond the body. The allantoidian veins (PI. 18a, 
fic. 7, 7, fig. Ta, 2, fig. 8°22, fig. 10, 72) are remarkably wavy in their course 
along the sides of the body, and even to the point where they empty (Pl. 18a, 
fig. 7, 7, fig. 8, 7, fig. 10, ct) into the venous sinus. The omphalo-meseraic or 
afferent vem (Pl. 18a, fig. 8, 7 fig. 9, 2 fig. 10, 7) empties, with the abdomi- 
nal, the cephalic, and the allantoidian veins, into a common reservoir, the venous 
sinus (fig. 8, 7, 10, 2). Its course near to and within the body is in contact with 
and along the lower surface of the intestine, (Pl. 18a, fig. 8, 7’, fig. 9, 7’,) and 
upon the upper surface of the liver (fig. 8, 7, and 9, 7). 
The intestine (Pl. 9e, fig. 8, x’; Pl. 18a, fig. 7a, n', fig. 8, n’, n,n’, fig. 9, n', n', n°; 
Pl. 24, fig. 2, 1°, n*) has become longer than the whole abdominal region, and _pro- 
jects in a fold through the ventral opening. It still remains in open communi- 
cation with the yolk, but, with a narrow aperture at its posterior end, (Pl. 18a, 
fig. 8, n°,) projects slightly beyond the body, but does not open so as to form 
