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18 BENNET M. ALLEN 
that has been lifted off the yolk, is decidedly shorter than in any 
of the 147 hour specimens. 
Amia, 182 hr. stage. In four specimens of the 132 hour stage, 
the caudal end of the embryo was just ready to undergo separa- 
tion from the yolk. Only in one of them had this really com- 
menced, the separated portion having reached a length of but 
20u. Not one of these four specimens showed a single sex-cell in 
the mesoderm. ‘There can be no question upon this point because 
they could be very readily detected if present. In the 137 and 
147 hour stages those that migrated into the mesoderm stand out 
most clearly and sharply from the surrounding mesodermal cells. 
The points of difference between the two kinds of cells are very 
striking and unmistakable. The sex-cells on the one hand are 
large, spherical, have sharply defined boundaries, and are filled 
with large oval yolk grains; while the mesodermal cells are small, 
flattened, syncytial, and contain a very few minute yolk granules. 
It is very much more difficult to trace the earlier history of the 
sex-cells in the peripheral entoderm, owing to the slight differences 
that may be taken as criteria in distinguishing them from the 
neighboring entoderm cells. Numbers of cells with all the 
characteristics of sex-cells are found just beneath the anlagen 
of the blood masses. This stage is just before the development 
of blood vessels within the embryo, and the blood-forming cells 
occur in the form of two sharply limited bands, one on each side 
of the embryo and at some distance lateral to it. Here and 
there, sex-cells are found in the peripheral entoderm, medial to 
these areas; but clearly defined cases of this sort are rather rare 
as compared with the large number seen in this region a little 
later in the 147 hour stage. It is quite likely that many of these 
sex-cells are overlooked at this stage owing to the fact that the 
neighboring entodermal cells contain rather large yolk grains at 
this time, while those seen in these cells in the 147 hour stage 
are much smaller than at this stage. 
It is quite possible that the sex-cells may migrate medially 
in the entoderm from an entodermal source beneath the blood 
anlagen to various points between this region and the edge of 
the sub-germinal cavity. It is possible that a large proportion 
