222 
think it unlikely that such is the case. This question is one that calls 
for further study. 
They continue to migrate through the endoderm, approaching 
nearer and nearer to the median line beneath the notochord (compare 
Figs. 2 and 3). They do not reach this position until the embryo has 
almost attained a total length of 5 mm as measured from the cervical 
bend to the caudal bend. The cervical bend is very pronounced at 
this stage. The mesentery is just being formed by the bending inward 
of the splanchnopleuric mesoderm. In Fig. 4, is shown a slightly later 
stage in which the sex-cells are 
seen to have not only reached the 
4 have begun to migrate upward into 
we 
Mes. som. Neph. 
Fig. 2. 
i 
1 
X 
A 
t 4 Ge SERS 
Neph. W.D. Neph. Mty- Sex-gl. W.D. 
Fig. 3. Fig. 4. — 
Camera lucida diagrams of the sex-gland region in three different stages, 
showing important steps in the migration of the sex-cells. 
Fig. 2. Embryo of 2,8 mm total length. X 82. Mes.som., Mesoblastie somite. 
Neph. Nephrotome — not yet clearly marked. 
Fig. 3. Embryo of 4,5 mm total length, 3,5 mm c-t length X 82. W.D. 
Wolffian duct. 
Fig. 4. Embryo of 4,5 mm, c-t length. X 82. Mty. Mesentery. Sex-Gl. Sex- 
gland anlage. 
the mesentery. I find myself at a loss to explain the means by which 
the sex-cells are enabled to perform this migration. It should be 
remembered that the period of time through the cells are migrating 
is quite considerable. Imperceptible changes in the shape of the sex- 
cells might bring about a slow amoeboid movenient sufficient to enable 
them to cover the distance in the allotted time. Whatever the means 
axial line of the endoderm, but to 
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