Cuap. I. TRANSFORMATIONS OF THE YOLK. 521 
found inclosing their mesoblasts (Pl. 9a, fig. 9, a, 4) after segmentation has com- 
pleted its purpose, and even at the time the cephalic hood has begun to form 
(fig. 34, d). 
many belonging to a particular heap, each mesoblast can be recoenized and distin- 
toro} 2 fo) 
Almost to the last moment before losing its identity as one of the 
guished from the waxy bodies, the entoblasts, which it usually incloses in greater 
or less numbers, (Pl. 9a, fig. 9, a’,) by the peculiar mesh-like condensation of its 
viscid contents upon the inner surface of its wall (fig. 9, a”). This may be seen, 
even in those which rival in minuteness the cells of a much further advanced 
embryonic dise. (Compare fig. 9, a’, with fig. 8). 
Not only the mesoblast, but the ectoblast also, gives the peculiar reaction formerly 
noted in regard to the cells of the interovarian egg, when water is applied; for, 
just as in these latter, the ectoblast swells up, and, finally bursting, after the trans- 
parent fluid contents in which the mesoblasts float have condensed into a swarm 
of minute oscillating particles allows them to escape, discharging at the same 
time its multitude of mesoblasts (PI. 9a, fig. 7, 7a). 
Going deeper into the substance, and more toward the back of the embryonic 
and the outer sur- 

dise — or, more properly speaking now, the back of the embryo 
face of the germinal layer, the heaps of mesoblasts become less and less distinct, 
owing to the closer application of the wall of the ectoblast against the mulberry-like 
surface of the mesoblasts, (Pl. 9a, fig. 4,) so that the mesoblasts of adjacent heaps 
interlock with each other to the confusion of the outline of each mass. Finally, 
almost at the outer surface of the yolk, the ectoblasts have disappeared entirely, (PI. 
9a, fig. 5a,) though the mesoblasts still remain in heaps, with more irregular outlines 
than is usual. The disappearance of the ectoblasts is so gradual, so imperceptible, 
that we have good reason to believe that they are slowly disintegrated and lique- 
fied, the result mixing with the surrounding fluid. The now free heaps of meso- 
blasts extend their boundaries in an irregular manner, inosculating with each other 
by the intermixing of their most superficial components (fig. 5, 6). Even here 
the mesoblasts retain their entoblasts, sometimes to the number of three or four 
in each, (fig. 5a,) and withal exhibit their vitelline character. The same may be 
occurs not only on one side, but all over the surface, is said to occur in Birds. By commencing our inves- 
of the yolk, for the very reason that these identical 
“seoment balls” are found upon the whole superficial 
extent of the ege, (Pl. 9a, fig. 4, and 34, 0,) and, to 
a certain depth, inwardly. But we think this total 
segmentation may be proved upon totally different 
premises, so that what we have just said above may be 
left for the consideration of those who would hold both 
to the partial segmentation of the yolk and to the 
development of a wall around the segment masses, as 
66 
tigation of the subject with these “segment balls,” and 
tracing their development in a retrograde series, be- 
ginning with Pl. 9a, fig. 9, and receding through fig. 7, 
4, 34, b, 36-36¢e, 37-37d, 38—38¢e, 40-401, we find that 
the first steps toward their formation are taken in the 
midst of the great yolk mass, the very spot from which 
segmentation is excluded by the advocates of partial 
and superficial segmentation in the classes of Birds 
and Reptiles. 
