1904.] NATURAL SCIENCES OF PHILADELPHIA. 385 
rently to be explained in at least its first manifestations as the direct 
result of increase in growth of one side over the other. After 4a? lies 
considerably more posterior than the derivatives of the large cell, 
which before lay opposite it (4c, 4c”, fig. 81), the cell 4b? is seen to 
be undivided as yet and still at the anterior median point of the 
enteron, showing that the change of position of 4c? relative to its 
opposite cell has been the result of greater increase in the area of the 
left over that of the right enteric wall. 
During this process 4a” has not been observed to divide and it main- 
tains its large size throughout. On the opposite side 4c? has divided 
into cells of equal size and divisions are continued in this region, result- 
ing in the thinning of that portion of the enteric wall and an equaliza- 
tion of the size of the cells which compose it. With the continued 
growth of the enteron 4a? is moved still more posteriorly and finally 
toward the right (left of figs. 82, 83). In fig. 84, which represents 
the enteron in optical section at a stage about corresponding to fig. 104, 
4a? is seen lying directly in the median line. Above, in the anterior 
median portion of the enteron, is a group of large yolk-ladened cells 
which have been derived from 4b? and its neighboring cells. This 
group will soon shift somewhat to the left and become the rudiment of 
the liver. 
As was seen before, the small cells E', E’, e', e?, which were separated 
from the anterior end of the mesentoderm, at first lie between 4a? and 
4c?. An actual section at this stage parallel to the ventral surface 
(fig. 85) shows that the inner of these cells are yet in contact with the 
enteric cavity. I am confident that the cells in this figure marked 
““enteroblasts”’ represent mesentoblastic derivatives. Their history, 
position, size and the structure of their nuclei, which are small and 
darkly stained, correspond to these cells. With the increase in extent 
of the left side of the enteron and, after the closure of the blastopore, 
by its continued growth, these enteroblasts, which may be distinguished 
from their neighbors by their darkly staining nuclei and their smaller 
size, become pushed from the median plane toward the right side as the 
large cell 4a? advances around to a more and more posterior position 
(fig. 83). Finally, when 4a? itself lies on the median line, these cells lie 
entirely to the right and are more posterior than those which have come 
from 4c and 5c. A slightly diagonal actual section, as fig. 86, shows the 
large cell 4a? in the median plane. Just behind it and slightly to the 
right are shown in the section five small cells lying closely pressed 
between 4a? and the shell-gland invagination behind. These cells 
correspond in position and in appearance to the small enteroblasts 
