334 WHEELER. (Vo. ITI. 
largest cells with the largest nuclei in the egg (Fig. 30 v). The 
long pseudopodial continuations of the finely and evenly gran- 
ular cytoplasm can often be traced for a considerable distance 
between the yolk polyhedra. The yolk cells are never seen 
in process of division, and as their number in eggs of widely 
different stages is approximately constant, I conclude that they 
rarely or never divide. 
The thickening process which formed the ventral plate still 
continues in a spot about one-fourth the length of the egg from 
the caudal end, and gives rise to a rounded mass of cells which 
are much thicker than the surrounding portion of the ventral 
plate though but slightly raised above the general surface (Fig. 
41). <A faint depression appears in the centre of the rounded 
mass (4f)._ While this thickening is forming, the nuclei of the 
ventral plate are also proliferating very rapidly at two points on 
the ventrolateral edges about one-fourth the length of the egg 
from the cephalic end (Fig. 41 pct). These cells also increase 
in depth, but do not rise above the general surface of the egg. 
The two patches of thickened cells are the precursors of the 
procephalic lobes. The deceptive appearance of a groove is 
presented by the keel (cz), which runs the whole length of the 
ventral face, but soon disappears as it is absorbed by the young 
embryo. 
The formation of the mesoderm can be traced in eggs sec- 
tioned during or shortly before the stage figured in Fig. 41. 
Figure 38 is a longitudinal section through the posterior portion 
of the egg (Fig. 41) through the middle of the thickened mass 
of cells and the depression 6p. In this egg the mesoderm has 
been forming for some time. The ventral-plate cells are col- 
umnar at 0g, and their nuclei are elongated in just the opposite 
direction to their former longest axis, which is still the longest 
axis of the serosa nuclei at sx. The depth of the ventral-plate 
cells gradually decreases anteriorly. The stellate yolk cells are 
scattered at various distances from one another under the ven- 
tral plate. The mesoderm (sd), as is clearly seen from the 
section, arises partly from proliferation of the ventral-plate cells 
under 4f and passes forward as a single but incomplete layer 
of cells. Towards the head this layer splits, and each of the 
two bands thus formed continues forwards under one of the 
procephalic thickenings. This is seen in Fig. 31 from a sec- 
