Or 
=~I 
ile) 
Cuar. I, FORMATION OF THE ORGANS. 
SOLON Yy Vir: 
FORMATION AND DEVELOPMENT OF THE ORGANS. 
While tracing the general progress of the development of the young Turtle, we 
have described with sufficient fulness the mode of formation and the earlier meta- 
morphoses of many of the organs;' and it would therefore be useless to repeat here 
many of these descriptions, now that we are about to consider the different organs 
and systems of organs separately. It will be sufficient for our purpose to refer to 
the figures and pages, in which much that might be required here has already been 
said, and then to fill up whatever blanks may be left, in order to illustrate the 
serial development of each organ. As we proceed in the description of these 
organs, we shall refer to the pages where they are mentioned in connection with 
the whole embryo; and in this way every student may be enabled, not only to 
follow the progressive growth of each organ, but also to appreciate the relations 
which every one bears to every other, at each successive stage of its special 
development. 
The Bran. The formation of the primitive furrow, which is the earliest step 
taken to lay the foundation for the brain and spinal cord, has already been 
described (p. 543, Pl. 9e, fig. 3, 4, fig. 3a, 6, fig. 4, 0, fig. 4a, 6; Pl 11, fig. 3, 
b, fig. 4, 5, 6). This primitive furrow soon becomes a deep gutter, of variable 
width and depth. In the head it is a very broad channel, from the beginning 
of its formation; but soon the germinal layer rises on each side of it in the 
form of a long ridge, in consequence of which the channel (Pl. 12, fig. la, e;. 
Pl. 9d, fig. 1, e', e&) is deepened. Gradually the ridges rise higher and higher, till 
the channel is as deep as it is broad; they then fold inwardly toward each other 
till their edges meet, first at one point, (Pl. 9d, fig. 1, ¢%,) and finally along the 
whole length of the head, (Pl. 12, fig. 3, ¢’,) thus forming a large, closed cavity 
(see p. 547). At this period, the cavity occupies the whole breadth of the head, 
as well as its whole length; the vertebral layer not being developed, except along 
the lower side of that organ (see Pl. 9d, fig. 1, /’). 
At the posterior end of the head, the brain gapes broadly open (Pl. 12, fig. 
8, e*, fig. 9, ¢, fig. 9a, ¢) for a considerable distance, as far back as just above 
the heart, where it again closes over rather suddenly as it meets the spinal tube 
1 Comp. p. 543-578. 
