SEX-CELLS OF AMIA AND LEPIDOSTEUS 19 
of them may have developed in the peripheral entoderm through- 
out this entire extent. On the other hand, it is also possible that 
sex-cells may migrate up into this region from the central ento- 
derm beneath. 
We have traced the history of the sex-cells from the 4 mm. 
stage where they are readily identified by any one who has had 
any experience in observing these cells, back to the earliest stage 
at which they are distinguishable in the entoderm. We shall 
now follow them up to the period when they are enclosed in the 
definitely formed sex-glands and finally to the stage at which 
they are found to have begun to increase in number. 
Amia 5 mm., total length. Passing from the 4 mm. stage to 
the next represented in our series, 5 mm., we find that the sex- 
cells have made but little progress in their migration toward the 
median edge of the lateral mesodermal plates. The total number 
of sex-cells counted in this stage was surprisingly small, being 
43 as compared with 87 in the 4mm. stage. This difference in 
number is probably due to individual variation. The hind gut 
has materially lengthened, being 1.3 mm. in length, compared 
with .88 mm. in the 4 mm. stage. There has been a corre- 
sponding increase in the length of the region over which the sex- 
cells are distributed. In the 4 mm. stage they extend from a 
point 0.06 mm., in front of the beginning of the hind gut, cau- 
dally to a distance of 0.35 mm. In the 5 mm. stage that we are 
considering, this region begins at the same point relative to the 
hind-gut and extends caudally for 0.50 mm., one isolated sex- 
cell being found at a distance of 0.57 mm. behind the cranial limit 
of their distribution. 
In the more caudal portion of this region the splanchnic and 
somatic layers of mesoderm have begun to separate to form the 
coelome. This separation does not at first lead to the formation 
of a continuous cavity but rather to a series of isolated, some- 
what rounded cavities. Further caudad, the coelome becomes 
more and more completely developed, appearing as a large cav- 
ity on each side. 
Amia 6 mm., total length. At this time the first sex-cells appear 
in the splanchnopleure just at the entrance of the hind gut. 
