B22 LANGENBECK. [VoL. XIV. 
Mesoderm. 
At the time when the entodermal invagination takes place 
the mesoderm is completely laid down and the appendages 
have begun to pinch off, each ridge containing mesoderm cells, 
as described above and figufed in Figs. 36, 37, and 55. 
To judge from what takes place in the great majority of 
other Crustacea heretofore described, we should naturally ex- 
pect to find the mesoderm arising from the region where the 
anterior lip of the entodermal invagination will be formed. In 
Microdeutopus, then, the extreme posterior end of the ventral 
plate would be the region where we should look for a prolifera- 
tion of mesoderm cells, since that is the region of the ento- 
dermal invagination. However, the posterior end of the 
ventral plate is exactly the region where the smallest number of 
lower layer cells are found; in fact, except for the few dorsal cells 
which were overgrown, that region is composed of only one 
cellular layer, whereas the head region at the time is composed 
of two or even three layers, and patches of cells are found 
irregularly scattered under the ventral plate, decreasing in size 
and number as they approach the posterior end. 
When the protoplasm rises to the surface of the egg to form 
the blastoderm the four cells of the ef group were found 
under the cells of the EF group. The dorsal cells, which 
are the descendants of the remaining micromeres, are over- 
grown by the ventral plate, and, subsequently, form part of the 
lower layer of cells. In Fig. 18 only the dark nuclei of the 
dorsal cells can be seen, their protoplasm being spread over a 
large surface, making such a thin layer that it cannot be dis- 
tinguished. As the ventral plate grows over the egg the proto- 
plasm of the dorsal cells concentrates around their nuclei, and 
the cells present the stellate appearance shown in Figs. 27 and 
34. Fig. 46 shows a section of the head region of an embryo 
at the stage shown in Fig. 30. A large cell (d) is seen just 
below the edge of the ventral plate. The beginning of the 
yolk area is shown at Y, and this area was greater in extent 
in the next section, showing that the cell above the large stel- 
late one is at the edge of the ventral plate. Fig. 48 also shows 
