338 WHEELER. . [Vov. III. 
GENERAL REMARKS. 
a. Germ layers. 
Our knowledge of the formation of the germ layers in the 
Orthoptera is still less satisfactory than in other orders of 
insects, as in most of the species studied even the formation 
of the mesoderm has not been clearly determined. 
Korotneff (25) failed to find the typical groove-shaped gas- 
trula in Gryllotalpa, and Ayers (1) had no better success with 
CGicanthus. On the other hand, Bruce (7) has observed the 
typical process of mesoderm formation in J/anézs, and Graber (15) 
in Mantis and Stenobothrus. Thus only representatives of the 
Orthopteran families W/anted@ and Acriditde are known to form 
their mesoderm in the typical manner. 
Nusbaum (33) concludes, on apparently very little evidence, 
that the mesoderm of //a¢ta is formed in the same manner 
as in Musca, and he gives a figure of a cross-section with an 
immense invagination. 
Cholodkovsky (10), in his account of the entoderm formation 
in Blatta, says that the gastrula which gives rise to the inner 
layer is very easily observed. 
As, I spent much time on a full series of stages between 
the formation of the ventral plate and the appearance of the 
appendages, without being able to find a trace of the elongate 
gastrula for which I was searching, I conclude that both Nus- 
baum and Cholodkovsky may have been deceived by the carina 
on the ventral surface, which, in reflected light, looks like a 
narrow groove. 
The method of germ-layer formation in AZaféa, at first sight 
so different from the method observed in Doryphora, may, how- 
ever, be traced to the typical process. My studies on Dory- 
phora make it probable that the entoderm of 4/atta originates 
in the mass of cells found under the area of proliferation, the 
more superficial cells of which grow forward as the mesoderm 
in a continuous median sheet bifurcating under the cephalic 
portion of the ventral plate. We shouid thus have a spot differ- 
ing in no essential particular from the caudal plate of Dory- 
phora. Jf we suppose that in A/atta the tendency to form the 
median groove has become very weak, and is now confined to 
