562 EMBRYOLOGY OF THE TURTLE. Part III. 
6, fig. 25,) so that in fact this is the widest part of the head, excepting the 
region which includes the eyes. 
Along a line just above the legs, both sides of the body project with a 
rather sharp edge, which is formed by a longitudinal fold in the skin, so that, 
passing from below upwards, the body suddenly broadens, and forms, above, as 
it were a roofing for the lower side of the animal. When seen from above, 
(Pl. 6, fig. 25,) this roofing appears narrow and ovate, with the broader end towards 
the head. This is the earliest period at which the shield commences to develop. 
The wavy surface of the back (Pl. 15, fig. 15) indicates the presence of the 
ribs, each wave corresponding to a single rib. The tail is also enormously 
developed, and more than equals the length of the trunk in Chelydroide and 
Emydoide; but in Chelonioide, Thalassochelys at least, (PI. 6, fig. 24,) it is not 
more than half so long. 
The eyes are much further developed, the pupil being now well formed in con- 
sequence of the perfect closing of the ring of the iris. The broad and deep depres- 
sions at the end of the head, mentioned in former pages, (p. 555 and 559; Pl. 24, 
fic. 12, »; PL 18a, fig. 9, v,) have, at this stage, (Pl. 24, fig. 11, v,) become very 
much constricted at the mouth, so as to leave a very small external aperture. Below 
these depressions there is a large opening (#) in the head, which leads into the 
beginning of the intestinal canal, and therefore must be the mouth. This being 
established, it is easy to see that the two depressions (v) above the mouth are 
the openings of the nostrils, and that the two fissures which run from the nos- 
trils to the mouth are each the remains of the posterior fold of the depression, 
The lower jaw has a distinct emargination at the middle. 
The region of the heart and liver still remains the deepest part of the body. 
The anterior part of the intestine, the oesophagus, (Pl. 18a, fig. 5, 7°.) opens 
with a nearly horizontal and narrow aperture; the stomach (x°) is still more 
bent upon itself as it winds around the left side of the liver (7); the long 
intestine (z') is more slender, when compared with the stomach and cesophagus, 
than heretofore, and still remains in open communication with the yolk sac. The 
respiratory apparatus (fig. 5, ¢, /') is more extended; the windpipe (/) is slender, 
and projects beyond the opening of the cesophagus (x’); the lungs (Pl. 18a, fig. 5, 7; 
Pl. 24, fig. 4) are divided into five broad compartments, or bronchioles, one of 
which (Pl. 24, fig. 4, 7) trends at right angles to the others, and in a horizontal 
direction as regards its position in the embryo. The epithelial lining (PI. 24, fig. 
4, 2) of the lungs is very much thinner than in the last stage, (PL 2A, fig. 3a, 2,) 
but the outer layer (1) is as thick as ever. The heart (Pl. 18a, fig. 5, h) 
retains the spongy nature of former phases; the aortic bulb (A') is larger. The 
efferent vessel, the omphalo-meseraic artery, (Pl 18a, fig. 4 and 5, j*,) springs now 

