602 EMBRYOLOGY OF THE TURTLE. Part III. 
8, fig. 34, a, p. 522). This is true, as far as their appearance is concerned ; 
but inasmuch as some are destined and tend to form one kind of organ, and 
assume features peculiar to the cells of that organ, and others form other organs 
and assume other very diverse features, it is impossible to deny that these appar- 
ently identical cells are at this time, respectively, very unlike in their intimate 
nature. The cells of the amnios, for instance, which we are about to describe, 
are totally different in appearance, at a certain stage, (PI. 9a, fig. 28,) from all the 
other cells of the body, and retain their peculiarities throughout the whole period 
of the existence of the organ to which they belong; and yet these cells were 
once apparently identical with all the other cells im the embryo. Thus, premis- 
ing that all the cells of which the embryo is composed have at one time a 
perfectly similar appearance, as far as our senses can perceive, we will proceed 
to describe the different changes through which these cells pass in the origin 
and development of the several organs. 
The Amuos. At the time the amnios is nearly closed over, its cells (Pl. 9a, fig. 
28) are arranged in a single layer. These cells are of moderate size, sharply polygo- 
nal, quite transparent, and nearly filled by a large, oval mesoblast; and the latter 
has perfectly homogeneous, clear contents, with a small, hollow, sharply defined 
entoblast in the centre. The mesoblast is not quite so transparent as the con- 
tents of the parent cell about it. These peculiarities are constant as long as the 
amnios exists, and by them the amnios may be recognized even at so late a 
period as when the Turtle is hatched; but then the cells are beginning to decom- 
pose, (Pl. 9a, fig. 31, 4, fig. Sla, 4,) and are slightly swollen. 
The Spinal Marrow. The cells of the spinal marrow were not made the subject 
of a particular study, except at one stage, a short time before the Turtle was 
hatched, (Pl. 15, fig. 1, 2, 3,) and then only in one part of this organ, namely, 
just behind the medulla oblongata. These cells (Pl. 19, fig. 21) are very similar 
to those of the medulla oblongata, (fig. 20,) but some are more hyaline. No cau- 
date cells were observed here, as in the medulla oblongata (fig. 20, a, a). The 
smaller cells are not mesoblasted. 
The Medulla Oblongata. Our investigations of the cellular structure of the 
medulla oblongata, and of parts of the whole brain, were not commenced ear- 
lier than in the case of the spinal marrow. At this stage, the cells (Pl. 19, 
fig. 20, a, 6) are irregular in outline, but more or less globular, and some have 
tail-like prolongations, forming the so-called caudate cells (a, a). The contents are 
light and finely granular, and surround a mesoblast, of variable size, which some- 
times almost fills the cells (4). 
The LHemispheres. The cells in this portion of the brain are variable in size 
and closely packed together, (Pl. 19, fig. 16b, 16c,) but not mutually compressed, 







