Nooza)  Ma(CRODEUTOPUS (GRVELOTALPA COSTA. 317 
although later they fall into line. In the isopods and in Mysis 
teloblasts give rise to regular rows of cells in the postnaupliar 
region. In Microdeutopus, however, I could find neither ecto- 
dermal nor mesodermal teloblasts. Bergh supposed that the 
regular arrangement of ectoderm cells, which is found in the 
amphipods, arose from some indistinguishable teloblasts, but I 
think this assumption is not necessary for Microdeutopus, since 
the regular arrangement is found also in the naupliar region 
(Figs. 27 and 31), where no teloblastic growth occurs. 
The dark patches (d@) in Figs. 28, 29, 32, and 33 represent 
cells below the surface, which, I think, have been overgrown 
by the ventral plate as it extends over the egg. In the isopods, 
as described by Dr. McMurrich, and in Mysis, as described by 
Bergh, the cells scattered over the dorsal pole are added to the 
ectodermal layer of the ventral plate as it grows over the egg, 
and with these facts in mind I searched carefully to see if this 
was the case with Microdeutopus. Further and careful exami- 
nation only strengthened the view that in Microdeutopus the 
dorsal cells are overgrown and so form part of the lower layer. 
At a little later stage (Fig. 34) the head region has increased 
in extent, a few ectodermal cells are beginning to differentiate 
at the sides, and the edge of the ventral plate is even more 
sharply marked off from the yolk than in the preceding stages. 
In the next stage (Fig. 30) the patch of dark cells has become 
very characteristic in appearance. The dorsal organ (d.0.) has 
begun to differentiate, and the outline of the ventral plate is 
no longer distinct. It appears as though a layer of protoplasm 
had spread over the yolk, and through this layer the nuclei are 
found irregularly scattered. This appearance is due, I think, 
to the lack of definite cell walls, for I have never been able to 
distinguish them in Microdeutopus. In the earlier stages the 
cells seem to be “raumlich centriert,’ as Flemming expresses 
it; therefore, in eggs of stages represented in Figs. 29, 33, and 
34, the cell boundaries can be distinguished, but in the later 
stages, where the cells become closely packed, cell outlines are 
lost. In eggs after the stage represented in Fig. 34, it seems 
as though the cells at the edge of the ventral plate have no 
longer the power to assume the spherical form, so making 
