8 BENNET M. ALLEN 
that the sex-cells undergo lateral migration, either apparent or 
real, so as eventually to lie at some distance on each side of the 
median point. 
The narrowest portion of the mesentery is at about one-quarter 
the distance from its origin to its insertion. Its minimum width, 
as measured here amounts to .028 mm., thus showing a great 
reduction as compared with the 10.7 mm. stage. This reduction 
in width is shared by the entire mesentery, certain regions remain- 
ing broad only on account of the enclosed blood vessels. No 
doubt the migration of the sex-cells out of the mesentery is in 
large part responsible for this, but a considerable share of it must 
be ascribed to the fact that there has been a tendency for the 
tissues to become more compact. 
The total length of the hind gut at this stage has reached 4.03 
mm. Sex-cells are found in the entoderm at its cranial end, and 
from there extend to within 0.62 mm. of the cloaca. The dis- 
tribution of sex-cells within the sex-gland anlagen is somewhat 
more restricted, since they extend from a point 0.31 mm. caudal 
to the beginning of the hind gut, to a point 1.00 mm. cranial to 
the cloaca. They are rather sparse at these two extremes. 
As in the preceding stages, there is no clear evidence of division 
of the sex-cells, although one can not be absolutely certain upon 
this point. While at this time many are free from yolk material, 
others show but little diminution of it. It is true that the sex- 
cells are often found arranged in clusters, but there is no evidence 
to show that these are due to repeated division of a parent cell 
rather than to a tendency for them to congregate through mutual 
attraction. What the nature of this attraction might be, we do 
not know; but it might well be akin to that influence which causes 
the sex-cells to migrate toward the sex-gland anlagen from their 
source. Similar clusters of sex-cells were found in early stages 
in Chrysemys. 
Lepidosteus 14.1 mm. total length. Little radical change is to 
be seen in this stage. The sex-cells were counted and gave the 
following results: 
