256 C. T. BRUES, 
and connected by protoplasmic threads (see Fig. 18). The connecting 
protoplasmic threads are extraordinarily thickened as can be seen in 
the figure. The peripheral protoplasm is very distinct also and seems 
to be thicker than in earlier stages, although the movement of the 
cleavage nuclei to form the blastoderm is not as yet apparent. As 
the bulk of the entire egg is rather small compared to the size of its 
nucleus, it is not surprising to find the cleavage nuclei connected by 
such unusually stout protoplasmic threads. 
The blastoderm arises as usual by a migration of the cleavage 
nuclei towards the periphery of the egg. All the cleavage nuclei 
take part in this migration so that none of them remain 
in the yolk from the first as vitellophags, for these 
latter do not appear until much later. The cells of the 
blastoderm never become closely applied to one another but always 
remain of a flattened, rounded-hexagonal shape and more or less ir- 
regular in size (see Fig. 7). As soon as the peripherally placed 
cleavage nuclei begin to round up in forming the blastoderm cells, 
the blastoderm begins to draw up on the yolk, leaving a circular 
blastopore-like surface where the yolk is exposed and comes in con- 
tact with the chorion. At the time when this exposed yolk surface 
‚has acquired a diameter of about 90 degrees, there are about 86 cells 
in the blastoderm as nearly as could be estimated from examining 
the eggs in section. No vitellophags are as yet visible in 
the yolk. 
The blastoderm continues to contract and draw up toward one 
pole of the egg. Thus there is a part of the yolk which is freely 
exposed since it does not remain invested in a Blastodermhaut like 
that present in the great majority of insects. Instead of the gastrular 
furrow originating from the modification of a few blastoderm cells, it 
arises from the entire blastoderm, for after continued contraction the 
cells of the blastoderm have all reached one pole where they form an 
irregular mass which sinks beneath the yolk somewhat, forming a 
germ band of the immersed type. There are at this point indications 
of a Blastodermhaut or serosa enveloping the yolk, but it is ex- 
tremely thin and delicate and disappears entirely during the later 
stages (see Fig. 8). This flattened oval irregular mass of cells soon 
begins to become more definite in shape and in the arrangement of 
its cells. All the nuclei tend to become arranged about the periphery 
of the mass and it assumes the form shown in Fig. 9. A still 
further symmetrical arrangement of the cells leads to the condition 
