No. 2.] BLATTA AND DORYPHORA. 345 
ogy makes such a supposition probable), we have a form which 
will unite readily with an embryo Myriopod in a corresponding 
stage of development. In Fig. 3 I have given a diagrammatic 
longitudinal section of Geophzlus about the time the appendages 
appear. Evidently either the excessive growth in length of the 
annelid-like body has necessitated the complete invagination of 
the embryo into the yolk, bringing the caudal and cephalic ends 
together, or this process has been adopted as a means of bring- 
ing the surface of the embryo into more complete contact with 
the yolk. It is only necessary to reduce the posterior half of 
the infolded embryo (included between 4 and »m in the figure) 
to a thin membrane to reach the condition of Ca/opteryx (Fig. 4). 
The membrane resulting from the attenuation would be the 
amnion. Will has emphasized the fact that the stage with the 
amnion almost as thick as the ventral plate with which it is 
continuous, still occurs in the ontogeny of insects (compare 
Llatta and. Neophalax). He suggests that the attenuation of 
the posterior half of an embryo like Geophilus to form the 
amnion may account for the great disparity in the number of 
segments between the Hexapoda and Myriopoda. 
A further difference is observable between Calopteryx and 
Geophilus. The end of the original caudal extremity in the 
former is joined to the anterior end of the ventral plate, thus 
closing the sack whose anterior wall is the ventral plate and 
whose posterior wall is the amnion. This sack is attached at 
one point to the serosa enveloping the egg. The union of the 
membranes to close completely the amniotic cavity is the hinge 
about which the further explanation turns. 
It seems essential in all the insects so far studied excepting 
Musca, where the envelopes are rudimentary, that the amniotic 
cavity should be shut off from the space between the vitelline 
membrane and the surface of the yolk. The reason for the 
closure is apparent if we regard the amniotic cavity as a 
place for the temporary deposition of excreted matters, as an 
organ functionally analogous to the allantois of higher animals. 
It has often been observed that the amniotic cavity of insects 
soon after its formation becomes filled with a clear liquid which 
during and after revolution is found as a much vacuolated coagu- 
lum about the feet of hardened embryos. It seems probable that 
while the inner ends of the ventral-plate cells are absorbing and 
