470 EMBRYOLOGY OF THE TURTLE. Part ID. 
the more central material. In this state, very little pressure is required to crush 
into angular fragments the now brittle shell of the mesoblast, and, by a trifling 
disturbance of the yolk fluid, the broken parts may be made to roll over till 
they display their inner surfaces covered with ridges, (Pl. 9, fig. 12, d, d',) or 
present a profile of their thickness, showing in an indisputable manner that this 
thickness is uniform throughout the whole extent of the spheroid mass, to which 
it bears a very small proportionate diameter. Now, the very fact that the water 
passes through the exterior of the mesoblast, leaving it intact, and powerfully 
reacts upon that which it last comes in contact with, is of itself evidence enough 
to show that the acknowledgment of a cell wall here depends merely upon our 
interpretation; and whether a denser layer, inclosing a more fluid substance, can 
be called a wall, or is to be considered only as the extreme of a mass grad- 
The latter 
view, fortunately, cannot be sustained without the help of questionable reasoning, 
ually increasing in density centrifugally, from centre to superficies. 
when we refer to the manner in which the entoblasts, as they gorge the parental 
matrix, press outwards, in angular prominences, (Pl. 9, fig. 6a, ¢, d, 7, 7,) the thin 
resistant layer which bounds their field of development, and persist in restraining 
them from projecting uncovered into the hyaline fluid of the primary cell. 
Again: the existence of entoblasts, without a cell wall to contain them, would be 
an unprecedented phenomenon; yet here such an envelope is denied, inasmuch as 
there does not appear any visible differentiated layer of protem compound corre- 
sponding to the usually received definition. But, as it has been shown regarding 
the wall of the primary cell, (which wall, as likewise here, is not visible at first on 
account of its lack of refractive powers,) there is a thin stratum, sufficiently tenacious 
to restrain the more fluid contents, and this stratum sustains a very different reaction 
from the latter when immersed in water. To such an envelope, whether visible or 
not, most certainly the title of cell wall belongs; and under this mode of con- 
sideration we may extend the definition of a cell wall beyond the hitherto  stere- 
otyped bounds, and embrace a broader and more general view of its essential 
nature, characterizing it as a hollow, more or less spherical, layer, of indefinite den- 
sity, tenacity, and refraction, which surrounds the field of some definite, though 
isolated and homogeneous, function. 
When speaking of the plasticity and resilience of the ectoblast, we have already 
1 As the entoblasts of the yolk cells have gener- 
ally been described as crystalloid bodies, swimming 
either free in the yolk or surrounded by a transpar- 
ent cell wall, (J. Mtrier, Ueber den glatten Hai 
des Aristoteles, Ak. d. Wiss., Berlin, 1842, p. 37, 
and Rathke, Entwick. d. Schildkréten, p. 5,) and as 
the mesoblast which incloses them has been ovyer- 
looked by all previous observers, we have not been 
tired in accumulating proof upon proof, in order to 
show that they are, in every instance, actually in- 
closed in a sae, and that this sac, and not the erystal- 
loid body, is the mesoblast of the yolk cells. 
