Cuap. I. DEVELOPMENT OF THE YOLK CELLS. 471 
mentioned a similar peculiarity in the mesoblast; but it is proper to revert to the 
same subject in this place, especially as some more details are necessary in another 
point of view. It is to the homogeneous fluidity of the contents of the meso- 
blast that we would now call particular attention. 
Let any one glance at a quantity of yolk cells squeezing their way among 
each other, and observe the easy mobility of the entoblasts as they oscillate in 
the mass of now constantly changing, unstable shaped matrix, flying from side to 
side almost as if they were thumping about in an empty space, and then say, 
whether the mass within the mesoblast which surrounds the entoblast is a connatural 
fluid, equidense throughout, or, centrifugally denser, as the collapsing upon the cell 
wall by aqueous reaction might suggest. To the latter view we have no ineli- 
nation whatever; but to the former we must give our unqualified assent. We 
have characterized so particularly the movements of the entoblasts under disturb- 
ing influences, in order to render more prominent the fact that these bodies return 
with unerring certainty to their proper position at the centre of their parental 
domain, as soon as they are relieved from the contact of neighboring cells simi- 
larly affected. Whether this phenomenon is to be ascribed to the same centripetal 
power that influences the origin of the entoblasts in a central rather than a lateral 
position, we can only conjecture; but it seems natural to suppose that there must 
be some unknown relation between the two, and that perhaps the one may be the 
complement of the other. One word more in reference to the conduct of the 
mesoblasts, after water has burst the parent cell. Under such circumstances the 
mesoblasts become easily agglutinated to each other by the slightest pressure, and 
their parietes are totally obscured, so that one might suppose the yolk cells had 
contained immense irregular mesoblasts (Pl. 9, fig. 12, e, /). 
From the time of the origin of the mesoblast up to that age when the egg 
measures about one sixth of an inch in diameter, the mesoblast seldom exceeds the 
semidiameter of the ectoblast, (Pl. 8, fig. 23d; Pl. 9, fig. 6a, ¢ d,¢ f, g, h, 4 J,) 
and often falls short of even that extent, especially just after that exceptional 
state, when the rather minute cells, ectoblasts, rapidly amplify their mesoblastic 
progeny (Pl. 8, fig. 25b and 23c) till they are nearly filled with them, and then 
just as quickly outstrip them in increment of bulk. At the latter end of this 
stage the ectoblast begins to fail in maintaining its greatly superior size, (Pl. 9, 
fig. 6a, a, 6, fig. Td, 7e,) and the mesoblast from this time forward gradually 
encroaches upon the fostering matrix, till the latter, in a full-grown egg, is imper- 
sonated by a moderately thin layer, resembling, in its beautiful transparency, a 
halo about a cluster of golden brilliants. Thus terminates the career of the mes- 
oblast in yolk cells, as far as its activity in the life of an interovarian egg is 
concerned; but another more remarkable phase is still to be gone through before 
