16 BENNET M. ALLEN 
were noted two or three instances in which the sex-cells were 
migrating from the peripheral entoderm into the mesoderm. 
Amia, 3 mm., total length; 155 hours. Inaspecimen of 3.0 mm. 
total length, the free caudal portion has but recently separated 
from the vitelline mass, and has attained a total length of .56 mm. 
By comparison with a number of embryos of 132, 187, and 147 
hours old, the age of this embyro was estimated to be very close 
to 155 hours. This estimate was made by counting the number of 
sections passing through the posterior part of the embryo free from 
the yolk mass. Sufficient numbers of embryos were used to give 
a fairly accurate determination, there being seven specimens of 
the 147-hour, three of the 137-hour, and two of the 132-hour 
stages studied. 
TABLE 4 
The numbers of sex-cells in each were as follows: 
RIGHT SIDE LEFT SIDE | TOTAL 
LESBIAN 2 4 8 peda) ec 39 53 92 
Tass agen aves «2 ae 59 48 107 
There were 49 sex-cells counted in the 3 mm., 155-hour embryo. 
This, it will be seen, is decidedly below the average and yet the 
number is greater than that found in the 5 mm. stage and in 
the much later 16 mm. specimens. 
Only two of the sex-cells have migrated a very short distance 
along the lateral plate of mesoderm, beyond a point overlying 
the lateral boundary of the subgerminal cavity; the remainder 
of them all lie lateral to it. It will thus be seen that they show 
a much earlier phase of migration than that observed in the 3.5 
mm. embryo, not only as regards the number that have migrated 
into the mesoderm, but likewise in the distance through which 
they have travelled in their journey in that layer toward the sex- 
gland anlagen. 
Amia, 147 hr. stage. That there is a great amount of individ- 
ual variation in the rapidity with which this migration from the 
peripheral entoderm to the lateral plates of mesoderm is accom- 
plished may be readily seen by referring to the numbers counted 
