or 
Cap. II. FORMATION OF THE ORGANS. 59 
‘region. Soon, however, the subsidiary layer forms a wall around the cavity of 
the heart, partly by a direct hollowmg in its own thickness, thus constituting 
the lower wall, as we may see by the thinness of the layer at this spot (Pl. 12, 
fiz. 7, A, 7); and partly by elevating the edges of the channel and closing them 
over, so as to form an arch, or superior wall. Thus, in the beginning the heart 
is a simple, straight tube, (fig. 7, 2,) trending in the same direction with the axis 
of the body. The posterior end of the heart becomes connected, or rather con- 
tinuous, with a transverse channel (fig. 7, 7) which is formed in the same way. 
and the anterior end with two other channels (fig. 7, 7) of similar origin, which 
pass, one on the right, and one on the left, up along the sides of the head. 
Presently the heart bends upon itself at two different points; at one point, 
behind, it curves to one side, (Pl. 12, fig. 10, 2,) and at the other, before, down- 
ward, as it joins the branchial aorte (fig. 10, 7). About this time, too, it 
begins to swell in its middle region, (fig. 6, 4,) and curves outward; moreover, it 
has become separated from the subsidiary layer by a kind of longitudinal con- 
striction; but the rest of the channels of circulation remain as they originated. 
These curves grow stronger until there are three quite distinct chambers, one in 
front of the anterior curve, the aortic bulb (Pl. 18a, fig. 13, A'); another, the 
ventricle, (4*,) just behind, trending in an obliquely transverse and backward direc- 
tion toward the right side; and the last, the auricle, (43) running from right to 
left and obliquely forward and downward. The communications between these 
three cavities, and also between the last and the vena afferens, (7) are still very 
broad. The wall of these cavities, especially that of the auricle, (4°) has become 
very thick. At this time there is a complete system of efferent and afferent 
vessels throughout the body, so that the blood passes in a perfect circle from 
and to the heart. 
Another chamber is subseqtently formed, by a median transverse constriction 
of the auricle, making in all three distinct compartments, (Pl. 13, fig. 2,) communi- 
eating with one another by narrow channels in the heart proper, and a fourth, just 
in front of it, the aortic bulb. The relative positions of the three chambers become 
changed at this time: the anterior one, the ventricle, (Pl. 18a, fig. 11, A+, fig. 
12, h*; Pl. 24, fig. 7, A*,) falls to a lower level, which is the same, or nearly so, 
with that of the middle chamber, the left auricle (Pl. 18a, fig. 11, 4%, fig. 12, 4°; 
Pl. 24, fig. 7, 4°). The inversion of the heart goes on till the relative positions 
of the auricles and ventricle are totally opposite to what they were at first; the 
auricles (Pl. 24, fig. 9, 4?) being situated next to the back and directly above the 
ventricle, (fig. 9, A*, fig. 9a, 4*,) which occupies the ventral region, to the total 
exclusion of the other chambers of the heart. This is the position which the 
heart holds in the adult. The ventricle, (fig. 9, 24, fig. 9a, X',) at the same time 
