Cuar. I. FECUNDATION. 489 
clear cells; but here the lateral position of some of them, and their variable size, 
testify to their reality, whilst the application of water brings them out more strongly. 
Our figure gives a good idea of their appearance under this influence. 
We have been fortunate enough to recognize these cells in an egg which had 
already been laid as long as eighteen days, and in it they were seen undergoing 
selfdivision (Pl. 9a, fig. 27). A greater part of the mesoblasts were double, 
and more or less separated from each other; and here and there were those 
which, partially constricted, already contained two entoblasts. The size of these 
double mesoblasts is exactly that which would follow the division of a single meso- 
blast like those observed in the younger phases of the same kind of cells. The 
same transparency and angularity as we have formerly observed prevail here, so 
that their identity is beyond question. 
A longitudinal section of an embryo, of about the age of that represented in 
Pl. 12, fig. 1, shows that this layer (Pl. 9d, fig. 1,.....) follows closely every fold- 
mg and bending of the germinal layer, (PI. 9d, fig. a, a', a, a, and a®,) whether 
it be over the curved back of the “embryo,” or into the furrow which forms the 
incipient spimal tube, (Pl. 24, fig. 13a, e, e,) or close to its now very much 
depressed head, or backwards and upwards again with the folds of the amnios. 
In later periods, when the amnios is closed over, the embryonal membrane forms 
an imner lining’ (Pl. Qe, fig. 7)... .... ) to the amniotic sac; a portion is also inclosed 
within the spinal tube, as its approaching edges unite above; and, in a transverse 
section of this tube, totally shut, a thin film (PI. 9e, fig. 6,..... ) was apparent, but 
evidently undergoing a change, no doubt tending to resorption. That portion of 
it, however, which surrounds the whole yolk, remains distinct until the young animal 
is hatched; but in these latter days it is evidently decomposing, (Pl. 9a, fig. 31, a, 
3la, a,) at least its cells were more or less separated from each other, and their 
walls ragged, as well as ‘those of the mesoblasts. In some cells two mesoblasts 
were still visible. 
SECTION? V IT: 
FECUNDATION. 
Ever since I have known that our Turtles lay only once a year, I have been 
struck with the fact that the ovary nevertheless contains eggs of very different. sizes. 
I was led by this observation to inquire into the duration of the growth of the 
ovarian egos, when I further noticed that these eges appear in well-marked sets 
62 
