1904.] NATURAL SCIENCES OF PHILADELPHIA. 383 



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 Aa? lies 

 considerably more posterior than the derivatives of the large cell, 

 which before lay opposite it (4c-\ 4c", fig. 81). the cell ^h^ is seen to 

 be midivided 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 resiUt 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 Ac^ has divided 

 into cells of eciual 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 fine. 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 W, E\ e\ e\ which were separated 

 from the anterior end of the mesentoderm, at first lie between isx^ 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 4a2 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 4a2 and the shell-gland invagination behind. These cells 

 correspond in position and in appearance to the small enteroblasts 



