4 BENNET M. ALLEN 
Lepidosteus 7.3 mm. total length. In the single series of 7.3 
mm. embryos the sex-cells in general still occupy the ventral and 
lateral portions of the gut entoderm, having come to lie in the 
dorsal wall at only a few points, especially toward the cranial 
end of the hind gut. A very few sex-cells are found to have 
migrated into the loose mesenchyme between the gut entoderm 
and lateral plates of mesoderm, occupying positions in it lateral 
and dorsal to the latter. These migrant cells are merely the 
precursors of a general migration which does not become con- 
spicuous until the embryo has reached a length of 8.5 mm. 
TABLE 1 
Number of sex-cells in Lepidosteus 
NUMBER 
STAGE TOTAL 
Entoderm Mesoderm 
mm | 
6.8 No count | 15 
8.6 to 73 
O52 os 136 
9. 3A ce 41 
9.3B cG 311 
973 '€ ce | 425 
10.7 133 | 674 807 
| a priiis | Root R. L. 
12.0 125 104 | 163 180 179 751 
hall 128 Re SEs ON 37 197 153 737 
17.0 No count | | 262 235 
18.0 Je | | ial 173 
24.0 af | ee ate / 154 
 ‘Int.—Intestine. Los R.—Right sex-gland. 
Root—Root of mesentery. L.—Left sex-gland. 
Lepidosteus, 8.6 mm. total length. Passing over several interme- 
diate stages studied, the conditions found in aspecimen 8.6 mm. 
long may be described. At this stage the lateral plates of meso- 
derm are just beginning to split and to form the coelomic cavities 
(fig. 7). The interval between the plates is filled with loose 
mesenchyme, of which that portion lying between the gut ento- 
derm and the aorta will later be condensed by the apposition of 
the lateral plates of mesoderm in formation of the mesentery. 
