SEX-CELLS OF AMIA AND LEPIDOSTEUS 3 
dorso-ventral dimension of .24 mm. and a transverse diameter 
of .20 mm. 
Lepidosteus 6.8 mm. total length. In a similar part of the hind 
gut of a specimen 6.8 mm. in length the dorso-ventral dimension 
of the gut endoderm is .084 mm. while the transverse dimen- 
sion is .056 mm. The total length of the hind gut in this later 
stage is 1.70 mm. as compared with .76 in the 4 mm. embryo. 
It is seen that there has been a very decided diminution in the 
diameter of the gut, and, furthermore, that this is out of proportion 
to the increase in length. It is correlated with a thickening of the 
gut wall, due to the drawing together of the component cells. 
In the 4 mm. stage the gut entoderm was composed of a single 
layer of cells, while in the 6.8 mm. stage under consideration its 
lateral and ventral portions are made up of two, and in some places 
three irregularly arranged layers of cells, while the dorsal wall 
is made up of a single layer as in the 4 mm. stage. Two series 
chosen from several of this stage may be taken as showing typical 
differences. Both show an advance over the preceding stages 
in the greater ease with which the sex-cells may be distinguished. 
This is due to the contrast between the ordinary entoderm cells 
in which a considerable amount of the yolk material has been 
absorbed and ‘the more rounded yolk-filled sex-cells. In neither 
embryo has the process of sex-cell migration commenced. This 
is clearly evident in one, while in the other there are a very few 
scattered yolk-filled cells of rather problematical character in 
the loose mesenchyme above and at the sides of the gut entoderm. 
One striking individual difference between-these two specimens 
is found in the fact that while in one the sex-cells have retained 
their primitive position in the ventral and lateral portions of the 
gut entoderm, in the other they have migrated up into its dorsal 
portions. Although it is somewhat difficult to establish with 
absolute certainty this migration from the ventral to the dorsal 
portions of the gut entoderm owing to the difficulty of distin- 
guishing the sex-cells in the preceding stages, the individual 
differences in this regard observed in this stage, together with 
the fairly reliable observations upon earlier stages seem to point 
to an actual migration of this character. 
