330 WHEELER. (Vor. Tit: 
and caudal ends (oe and #) passes imperceptibly into the much 
thinner amnion, is considerably thickened and has its crowded 
nuclei in several rows, though but one row of cells is present. 
The depressions which mark off the incipient appendages are 
deep and narrow. The mesoderm (msd) is spread out under 
the whole of the ectoderm and has begun to thicken under 
each somite preparatory to segmentation. It is very noticeably 
thickened in two places: under the stomodzeal depression (f/!) 
and under the caudal plate (f/?), where it forms a large mass of 
cells projecting into the as yet unsegmented yolk just beneath it. 
These two masses of cells are the independent sources of the ento- 
derm, which grows backwards as two strings from the anterior 
mass (pl), and forward as two strings from the posterior mass 
(p/*). As we shall see further on, these two strings unite near 
the middle of the body and then begin to grow at their lateral 
edges till the mesenteron thus formed incloses the yolk. 
The points from which the chords grow are plainly seen in 
the figure (ez¢! and ent?). Under both points of proliferation 
there are a number of nuclei which at first sight under a low 
power seem to be dividing karyokinetically. The chromatin is 
all aggregated in one or two dense masses in the hyaline karyo- 
chylema, and thus resembles the similar aggregations seen in 
kinetic nuclei. These nuclei, however, are not dividing, but 
undergoing decomposition, as we shall see when we come to 
examine a more highly magnified section through the caudal 
plate. 
Before leaving Fig. 82 I would call attention to the three cells 
at c which are on the surface of the embryo in the amniotic 
cavity. They are very large and clear, and the more anterior 
is apparently creeping in the manner of an Amceba along the 
surface of the abdominal ectoderm. These cells, the ultimate 
fate of which I have been unable to determine, probably escape 
from the anal orifice of the gastrula before it closes. I have 
‘im several cases seen such cells issuing from or still in connec- 
tion with the infolded pocket of ectoderm, which is called meso- 
derm as soon as the outer layer has closed over it (Fig. 87 c). 
These peculiar cells may be the homologues of the ‘ Polzellen’ 
long ago observed in certain Diptera. 
A much clearer understanding of the method of formation of 
the entoderm may be obtained from Figs. 87 and 88, both rep- 
