SEX-CELLS OF AMIA AND LEPIDOSTEUS 31 
definite, rounded outlines and fine chromatin network, but by 
their large yolk content and the fact that they do not divide 
during the stages in dispute. _ 
The sex-cells are migratory to a high degree. The path and 
time of their migration may vary greatly within a given group 
of animals, as illustrated by the case of Amia and Lepidosteus. 
While in the forms that I have studied they are first to be ob- 
served in the entoderm, I am quite open to conviction that in 
other forms they may migrate from this layer into the potential 
mesoderm before the two layers are separated, as shown by 
Wheeler in Petromyzon. It is even conceivable that they may 
lie, from the very beginning of development, in material destined 
to form mesoderm—that they may never have existed among 
cells actually or potentially entodermal. The more recent de- 
velopment of our work along these lines, however, most cer- 
tainly tends to show that it is usual among the vertebrates for 
the sex-cells to first appear in the entoderm. 
SUMMARY AND CONCLUSIONS 
1. The sex-cells of both Amia and Lepidosteus have their 
origin in the entoderm. In Amia they are first distinguishable 
in the peripheral entoderm from the lateral angle of the subgermi- 
nal cavity to the anlage of the blood cells. 
In Lepidosteus they are first seen in the ventral and lateral 
portions of the gut-entoderm, although analogy with Chryse- 
mys leads us to assume that they may have migrated through 
the entoderm to these regions from more lateral anlagen, similar 
to those from which the sex-cells of Amia arise. In both forms, 
the sex-cells arise only in the region of the hind-gut. None were 
found at any considerable distance in front of it. 
2. The path of sex-cell migration in Amia carries them out 
of the peripheral entoderm directly into the overlying lateral 
plates of mesoderm, along which they travel, to come to rest 
near the medial edges of the latter. These portions are destined 
to join above the intestine to form the mesentery. As the 
splanchnic and somatic layers of the lateral plates of mesoderm 
