Cuap. II. FOLDINGS OF THE EMBRYONIC DISC. 561 
The area pellucida, (Pl. 14, fig. 2, 2a; Pl. 18, fig. 8,) and the space to a con- 
siderable extent beyond it, are much more sunk than in the last stage, (p. 557, PL. 
16, fig. 6,) so as to correspond to the increased size and expansion of the allantois. 
The area vasculosa covers at least one half of the yolk mass (Pl. 18, fig. 8). The 
fork of the vena afferens, and the portion of the vena terminalis and of the omphalo- 
meseraic vessels next to it, (Pl 14, fig. 2a,) have sunk below the surface of the 
yolk mass, but yet not so far as to be invisible. In another instance (Pl. 14, 
fiz. 2) the vena afferens (7) itself; and at least one half of the vena terminalis, 
(7*, 7") have sunk considerably below the level of the vascular area, and retracted 
within its former boundary. What appears to be a circular vessel in the place 
of the one that is now depressed is the line along which the vessels of the area 
vasculosa bend upon themselves, and plunge into the yolk mass, in order to join 
the vena terminalis (77, 7"). 
The yolk sac has decreased considerably in bulk, and occupies about two 
thirds of the cavity of the shell, the other third being filled by the embryo and 
its envelopes. The lungs (Pl. 24, fig. 3, 3a) of an embryo four days younger 
than this are much more developed; their transverse diameter is much greater 
at the base than at the apex, so that they resemble in figure the adult state; 
and the cavity of each is subdivided into two compartments, (fig. 3a, 2, 2,) 
namely, the original one, running along its whole length, as in the last stage, 
(fig. 2a, 2,) and another, much shorter, which branches from the main chan- 
nel at the base of the lung, and trends toward the ventral side of the ani- 
mal. The epithelial layer, (fig. 5a, 2, 2,) which lines this double chamber, is 
much thicker than in the last phase, not only in the lungs, but also in the 
windpipe, (2”,) especially where the two bronchiew branch (5). The cavity of 
the lungs is as yet very narrow, resembling a mere slit along their length. 
Up to this period the head has exhibited moderate proportions when com- 
pared with the trunk of the embryo, so that its size has not attracted any par- 
ticular attention.. In the next stage, (Pl. 15, fig. 13; Pl. 16, fig. 3; Pl. 18a, 
fig. 4, 5; Pl. 24, fig. 11,) however, there appears a great disparity between the 
different regions of the body. The head has increased to an enormous size 
when compared with its former proportions, so that it almost equals the whole 
trunk; in fact, if an outline of the head and neck be laid over that of the 
trunk, the two will be found to be nearly equal in size. The breadth of the 
head, including the eyes, far exceeds that of the body, although the latter has 
broadened also. (See Pl. 6, fig. 25, which, although belonging to a totally different 
family from that of fig. 13, Pl. 15, is yet identical as regards the proportions of the 
body, so far as a small figure may serve for such a comparison.) The encepha- 
lon, just above the eyes, is much broader than has been noticed heretofore, (Pl. 
71 
