No. 2.] BLATTA AND DORYPHORA. 339 
the posterior end of the ventral plate, we may regard the de- 
pression 4p as the remains of the blastopore. I have remarked 
that the gastrula in Doryphora is much deeper posteriorly than 
anteriorly, and providing a tendency to obliterate the invagina- 
tion should become apparent in an equal degree throughout its 
whole length, a short posterior depression like that in A/atta 
would be the result. But the opposite possibility, viz.: the 
derivation of a gastrula like that of Doryphora from a circular 
form like that of A/afta, is likewise worthy of attention. Ac- 
cording to Sedgwick (43), the gastrula of Peripatus elongates 
with a concomitant closure of the median portion of its orifice. 
Of the two openings thus formed the anterior becomes the 
mouth while the posterior becomes the anus of the embryo. 
Providing a similar stretching of the gastrula has taken place 
in the ancestors of Doryphora and Hydrophilus, it would be 
easy to see how the cells of the original single Extodermanlage 
might be separated to form two masses, which now arise beneath 
the stomodzal and proctodzeal area, and how the formation of 
the mesoderm from the edge of what was once the gastrula lips 
might continue throughout the whole portion of the embryo 
between the mouth and anus. If this has been the true 
evolutionary process in the development of the elongate gas- 
trula of insects, it seems probable that A/atfa may represent 
figure 1.— Diagram of germ-layer formation in Doryphora. vp. ventral plate; 
og. oral end of gastrula; ag. anal end of gastrula; g. central portion of gastrula; en. 
entoderm; e. prolongations of entoderm; aa. plane of cross-section 4; 46. of cross- 
section B; cc. of cross-section C; ec. ectoderm of ventral plate; ms. mesoderm. 
figure 2.— Diagram of germ layer formation in Blatta (somewhat hypothetical). 
Letters the same as in Figure 7. 
