594 EMBRYOLOGY OF THE TURTLE. Part III. 
attention has chiefly been devoted to those points which are less fully illustrated 
in Rathke’s monograph. The minuteness with which he has described the mus- 
cular development of a number of young Testudinata belonging to several distinct 
families’ has induced us to enter into fewer details upon this point, especially 
since the muscles of the adults are fully described in a preceding part of this work. 
On that account, we have taken more particularly into consideration the Histology of 
this system, for which we refer to the next section. (See also p. 592, Pl. 19, fig. 25.) 
The Heart. The heart, and in fact the whole circulatory system, is a develop- 
ment of the dorsal surface of the intestino-subsidiary layer into a vast network 
of anastomozing channels, through which the blood runs in certain determined 
directions. It is within the boundaries of the central propeller of the circulation 
that the blood first makes its appearance, surging backward and forward. At first, 
circulation is a mere tossimg of elementary cells and albuminous fluid from one 
side to the other of the simple saccular heart (p. 547; Pl 12, fig. 7, 2). After 
the vertebral layer (Pl. 9e, fig. 4, f', fig. 4a, f’, 7°, fig. 4b, 71) has divided off 
from the subsidiary layer, (fig. 4, 0’, 2, fig. 4a, o', n, fig. 4b, o',) the latter becomes 
separated from the former to a considerable extent, so as to leave a cavity (PI. 
9d, fig. 1, A, y?; Pl. Qe, fig. 5, jy?) of variable depth between the two. This 
cavity is deepest along the median line of the body, and grows shallow on each 
side till its upper and lower walls meet along the sides of the embryo. At 
the anterior part of the body, the largest and deepest portion (Pl. 9d, fig. 1, 2) 
of the cavity is nearly altogether shut off from the rest; this is the heart. 
As yet there is no circulation in it, there being no walls proper; in the begin- 
ning it merely marks its position, just in the same way that the primitive fur- 
row indicates the site of the spinal marrow. However, in order to avoid con- 
fusion, we will at once designate it as the heart, and speak of it as such dur- 
ing its formation, as well as afterwards. In the beginning, the heart lies at the 
inner angle of the curve of the head, so that it is partly in the dorsal and 
partly in the ventral region, and is essentially more anterior than the brain (fig. 
1, e', e'). This latter feature would be more readily perceived if the embryo 
were represented in a straightened position. 
On account of the absence of a circulating fluid of any consistency, the 
heart is not perceptible in a general view of the whole animal. It requires a 
longitudinal section (Pl. 9d, fig. 1) of the embryo, in order to lay it open to 
view. On this account it is not seen in several of our figures representing 
external views of some older stages, (Pl. 12, fig. 5, 5a, 4,) where the anterior 
bend (fig. 3, a°, fig. 3a, a°) of the body has receded toward the abdominal 
? Rathke, Entwickelung der Schildkroten, p. 154. 2 Comp. Part II., Chap. 1, Sect. 7, p. 270. 




