6 BENNET M. ALLEN 
consequently much more clearly defined. In three specimens 
9.3 mm. long the following counts of sex-cells outside of the gut 
entoderm were made: 
A = 41 B = 3ll C = 425. 
A and C are extreme cases, indicating that the process of 
migration is an irregular one in point of time. The mesentery 
in specimen A is .46 mm. wide. 
In specimen C those sex-cells destined to migrate out of the 
entoderm have for the most part already done so, while in A, 
an embryo of the same stage, the process is just beginning. 
The coelome is least developed in A and furthest advanced in C. 
This would indicate that the extent to which the migration of 
sex-cells has been carried on is correlated with the degree of 
development of the mesentery, resulting from the enlargement of 
the coelomic cavities. While these three specimens belong, no 
doubt, to slightly different stages of development, they were very 
carefully matched as to length, and are most certainly of the same 
age. 
Lepidosteus 10.7 mm. total length. At this period of develop- 
ment, the mesentery is well formed, being muchthinner (.18 mm.) 
than in the 9.3 mm. stage. This results in its possessing a denser 
texture (fig. 9). The great majority of the sex-cells are scattered 
through the mesentery, showing no definite arrangement; but 
lying for the most part in the mesenchyme enclosed between the 
somewhat denser splanchnic layers of mesoderm. A few are 
found in the mesodermal layers of the intestine, while a fairly 
considerable number have remained in the gut entoderm. At 
this time such sex-cells as are found in any but the dorsal wall 
of the intestine, at its junction with the mesentery, are most 
probably destined to remain in their present positions. A few 
of the sex-cells have migrated to places immediately dorsal and 
lateral to the root of the mesentery. The latter may be con- 
sidered to have reached the sex-gland anlagen, although their 
position relative to the root of the mesentery will be shifted, as 
we shall see, in the later stages, probably by a general shifting 
of the tissues in which they lie. 
