Agassiz on the Embryology of the Turtle. 345 
absent in quite large cells; in fact, an egg little more than one- 
sixteenth of an inch in mean diameter contains numerous cells 
and unmistakable properties, not to be confounded with any 
other cell contents.’ 
No ectoblast, as these albuminous hyaline vesicles are called, 
which has attained to a diameter of z7sath of an inch, is with- 
outa mesoblast. This latter body, like the Purkinjean vesicle, 
originates as a conglomeration against the wall of the cell,—the 
eetoblast,—which directly encloses it, but as soon as it has be- 
come well defined in outline it breaks away from its attachment 
 tnuch as a cell wall has been found, here, in these investigations, 
to have no definite density and in some cases to be hardly differ- 
_ fitlated from the substance which it contains, it has been thought 
: best to extend the definition hitherto used, and characterize it as 
“a hollow, more or less spherical layer, of indefinite density, 
tenacity, and refraction, which surrounds the field of some defi- 
y the time the egg has attained ‘to its ultimate size, within 
the Ovary, the mesoblast has grown so large as to almost com- 
: ly fill the ectoblast. The size of the mesoblast, at this pe- 
~“, Is enormous, far exceeding in this respect the mesoblast of 
any other free cell known, being about ;1,;th of an inch in 
diameter. 
Perhaps th 
are the entoblasts of the yolk cells. These bodies appear 
bout the time the egg has reached from one-eighth to one-sixth 
«2 inch in diameter; a single one at first originates in the 
other cases, but thers take their places around the first. 
: The marked peculsesity, _ these bodies ms their sha angularity, 
fom the time of their origin, which increases till they resemble 
i Spleulie ; and eventually fill, to surfeiting, the mesoblast. Then, 
When the egg has reached a diameter of one quarter of an inch, 
these angular, crystalloid entoblasts begin to lose their corners, 
“conn SERIES, Vor. XXV, No. 75.—MAY, 1858 
44 
