596 EMBRYOLOGY OF THE TURTLE. Part III. 
that it sinks toward the ventral side, increases very much in size, so as to equal 
im bulk both auricles, and, moreover, undergoes a remarkable internal change, 
which consists in the formation of a thick network of high and narrow semi- 
partitions on its inner surface, occupying nearly the whole cavity of the chamber. 
The figures of the next stage, (Pl. 18a, fig. 8, 9, 10,) which, by the way, 
have already been sufliciently described, (p. 559,) apparently contradict the state- 
ments just made in regard to the relative size of the chambers of the heart; 
but this is readily explained by the fact, that, during the contraction of the 
ventricle, (fig. 8, 2%, fig. 10, 2%) the auricles (fig. 8, A, fig. 10, 2°) become 
very much distended and enlarged. Now that the heart has assumed a_ position 
which it ever after holds, the only further changes which it undergoes relate to 
the comparative size of the different chambers, the narrowing of their channels of 
communication, and the formation of valves at the mouths of these channels. The 
ventricle (Pl. 24, fig. 10, 10a, 10b, 4*) finally becomes twice as large as the two 
auricles combined (fig. 10, 10a, 10b, 4’); the meshwork of semi-partitions disap- 
pears, and is replaced by a very thick wall, (fig. 10b, 24) traversed by irregular 
canals. It can hardly be doubted, from the appearance of the wall of the ven- 
tricle, that the semi-partitions have become obscured simply by thickening until 
they touch each other, except at a few points, which correspond to the irregular 
canals which pass from the internal nearly to the external surface of the wall. 
The manner in which the valves (PI. 24, fig. 10b, 7) are formed has not 
been ascertained by tracing their development; but, as they are simple projections 
from the edge of the opening (fig. 10b, vo) of the passages from the auricles 
(1°, h®) into the ventricle, (/*4,) it is very probable that they originate by a 
gradual elevation of this edge until it constitutes a lappet, or fold, sufficiently 
broad to extend across the whole diameter of the opening. These two valves 
are properly within the ventricle, (fig. 10b, /*,) and, when inactive, hang loosely 
into its cavity; but upon the contraction of this chamber they are pressed upon 
by the blood, and apply themselves over the apertures to which they respectively 
belong, and prevent the regurgitation of blood into the auricles (fig. 10b, /°, 4°). 
The valves at the opening of the venous sinus (fig. 10a, vs) are mere protru- 
sions of the lips of a transverse fissure into the right auricle. The passage of 
the blood through the heart at this phase is very irregular; first entering at 
the venous sinus, (fig. 10a, vs,) it passes into the right auricle, and thence, in part, 
directly into the ventricle, (A*,) and in part into the left auricle, and from that into 
the ventricle, (/*,) thence again to pass out through the right! and left (fig. 10, 
1 The right aorta, unfortunately left by mistake from its point of origin, and may be recognized here 
unlettered in the figures, doubles at a little distance as the shortest vessel in the figure. 







