132 PATTEN. [Vov. XII. 
body of the embryo. The elevation forms a broad, figure-8-shaped ridge 
(marginal folds ?), with a slit-like depression in about the centre of each loop. 
There is a thickened layer of mesoderm beneath the whole of this axial portion. 
Beneath the anterior end is a very large solid mass of mesoderm cells, ¢.77.5., 
from which pseudopodia-like streamers of cells radiate deeply into the yolk. 
Fic. 73, X 60, sectioned. In this embryo the tissues are beautifully clear, 
and sharply differentiated, resembling histologically the late stages in the forma- 
tion of the blastoderm of normal embryos. The ectoderm, instead of containing 
flattened cells with little protoplasm, is composed of high columnar cells with 
nuclei at their outer ends, their inner ends being filled with masses of yolk gran- 
ules. The whole layer is sharply separated from the underlying yolk and 
mesoderm. 
There is no cluster of yolk cells at the anterior end. The dark median band 
consists of columnar ectoderm cells, like those described above, but higher, and 
below them is a clear band of mesoderm. Both layers are entirely undifferentiated. 
At what seems to be the posterior end, is a deep, oblong depression with two 
infoldings of its anterior wall, like steps leading down into it. Numerous de- 
generating nuclei arise from the inner surface of the infolded layer, and lie scattered 
about in the neighboring yolk. The mesodermic area is circular, with a faint 
thickening of its anterior and lateral margins. There is no trace of a posterior 
concrescence. 
FIG. 74, X 60, sectioned. Embryo similar to the preceding. It consists of 
two disc-shaped ectodermic thickenings, connected by a longitudinal band of the 
same nature. Beneath the thickened ectoderm are corresponding mesodermic 
thickenings, but especially enlarged at either ena. 
There is a shallow depression in the centre of each ectodermic disc. In both 
Figs. 74 and 76 the ectoderm of the posterior thickening shows a higher grade of 
histological differentiation than those of the anterior one. 
The peripheral outline of the mesodermic area was not visible. 
Fic. 75, X 60, sectioned. An embryo with a circular, mesodermic area, and 
a “blastopore ’’-like invagination at either end. 
The invaginations are similar to those already described, except that com- 
paratively few yolk cells surround the invaginations. The outer surface of the 
invagination is covered with a thick layer of bacteria (?). The ectoderm is a 
uniformly thin layer of cells, showing no trace whatever of a median thickening 
connecting the two invaginations. The mesoderm is remarkable in that it is of 
nearly uniform thickness throughout its whole extent, except at the margin where 
it is decidedly thickened, 7z.a. 
Most of the marginal mesoderm is composed of the characteristic, striated cells 
seen in stage D, and later. 
Fic. 76, X 60, sectioned. This embryo consists of a circular, mesodermic 
area containing scattered, star-shaped masses of cells, either extending from the 
mesoderm into the underlying yolk, or lying freely in it. There is.a deep, circular, 
ectodermic invagination at the anterior end with a thick layer of mesoderm sur- 
rounding it, and a similar, but larger, deeper, and more irregular cavity, at the 
posterior end. The ectoderm lining this invagination is considerably thicker than 
that of the anterior one, and the thickening extends along the surface for some 
distance back of the infolding, f.a.c. This thickening is probably the result of 
the concrescence of the posterior margins of the mesodermic area. There are 

