No. 2.] BLATTA AND DORYPHORA. 347 
(Fig. 8) may have originated in two ways: either from a sus- 
pended entoblastic embryo like Calopteryx by a separation of 
the amnion from the serosa at the point of suspension (Fig. 4 as) 
and the consequent passage of yolk between the two membranes, 
or from the ectoblastic type of A/atta by a separation of the 
amnion from the serosa throughout their area of contact, accom- 
panied by an intrusion of yolk. All that we at present know 
concerning the formation of the envelopes in Lepidoptera tends 
to prove that the latter method is the more probable. 
The hypothesis of Will and myself as set forth in the above 
paragraphs might be called a mechanical explanation as opposed 
to the views of those who see in the embryonic membranes 
rudimental structures like the remains of the trocosphere, larval 
skins, etc. Our hypothesis has at least the virtue of utilizing 
the facts near at hand. 
DESCRIPTION OF THE EXTERNAL CHANGES IN THE EMBRYO 
BLATTA AND DorRYPHORA UP TO THE TIME OF HATCHING. 
Blatta. 
Soon after their completion, the amnion and serosa become 
more attenuated on account of the flattening of their cells and 
the consequent diastasis of their nuclei. This thinning out of 
the envelopes permits a better view of the embryo and its form- 
ing appendages. 
On about the tenth or eleventh day from the beginning of 
development, the embryo presents the appearance represented 
in Fig. 45. The broadly rounded procephalic lobes are separated 
by a deep incision in the median line, and the antennz (a?) 
growing from the posterior lateral corners of the lobes have 
become prominent, while the backward direction of their growth 
is apparent. The labrum (/0) has appeared as a thick, crescen- 
tic and slightly divided fold in front of a faint depression 
which is the commencement of the stomodzeal invagination. Of 
the three pairs of oral appendages, the second and third (sz11, x") 
are clearly rounded and directed backwards ; the mandibles are 
still small. Each segment of the abdomen presents a pair of 
indistinct appendages. These subsequently disappear, with the 
exception of the pairs on the basal and terminal segments, 
which undergo a differentiation peculiar to themselves, As I 
