Cuap. I. DEVELOPMENT OF THE YOLK CELLS. 469 
39a— 39d)! Now it is in these latter stages that the development cf the meso- 
blast may most readily be mistaken for the primary cell genesis;—a well matured 
cell for one just formmg! Such an error however is excusable only in one 
who has taken but a glance at the yolk in certain stages of its development, — 
nay, hardly even then. But when the whole series of phases is followed with 
patient eye and thought, it is impossible to fail in recognizing the true and only 
prospective system of cell growth, the plan according to which each and every 
yolk cell has originated, advanced, and finally received the last touch, to fulfil 
the end for which it was intended, from the beginning, by its projector. 
It is only during the beginning of its life that the mesoblast preserves uni- 
formly any thing like a spherical shape. As soon as it has defined its outline 
clearly, growth ensues, and a more or less irregular, and very often angular, con- 
tour bounds its contents, throughout the interovarian life. As regards the last- 
mentioned shape, were it not for the peculiar reaction which water produces in 
its contents, causing a condensation in a network form, with less or greater 
meshes upon its wall, (Pl. 9, fig. 2a, a, 6, d,) it might be mistaken for an ente- 
blast; but the entoblast is not at all affected by such a reagent. Moreover, mes- 
oblasts with crystalline configuration are often met with, which contain entoblasts 
bearmg every characteristic of those observed in older phases of development 
(Pl. 8, fig. 23d, ¢, &, 7). However, it is only at about this age, when the egg 
measures from one tenth to one eighth of an inch in diameter, that such an 
unusual angularity of the entoblasts obtains; the subsequent stages, up to those 
of the full-grown ovum, are characterized by irregular oval or spheroid shapes, 
and, rarely, with here and there a perfect sphere (Pl. 9, fig. 6a, a, 6, e, g, and 
fig. 1li). Contemporaneously with irregularity of form it assumes also a change 
in color, till very soon, at about the most angular phase, it has passed from a 
famt to a dark yellowish tint, which it ever after retains as a characteristic com- 
plection, distnguishmg it from the brilliant, clear, golden yellow of the crystal- 
loid entoblasts. 
With reference to the reaction which water produces upon the contents of 
the mesoblast, we may add a few more remarks in detail, in order to bring the 
changes thus produced to bear upon the question of the existence, if not of a 
wall, at least of a denser exterior layer surrounding the entoblasts. Indeed, the 
presence of such a layer cannot be questioned; for, whilst it forms the basis 
upon which the contents collapse in wrinkles of coarser (Pl. 9, fig. 2a, d, 3a, ¢, d, 
12; a, 6, ¢) or finempfolds, (Pl. 9, fig. 2a, a@, 4, 3a, a, 5, g, Td, Te, 7f,) anasto- 
mozing with each other like the meshes of a network, it yet preserves exteriorly 
its form intact, except in some cases, where the cause of the shrinking within 
has distorted (Pl. 9, fig. 12, a, 6, ¢) the foundation upon which it has impelled 
