No. 3-] THE EMBRYOLOGY OF A TERMITE. 539 



That the under-layer is formed most easily and efficiently by 

 a process of invagination seems evident, from the almost uni- 

 versal appearance of the gastrula groove in insects. Given 

 first the more primitive, diffuse method of forming this layer 

 still persisting in the Termite and, as Heymons claims, in 

 other primitive insects, we may attribute to Natural Selection 

 its improvement until an invaginated gastrula groove has 

 become the common and readiest means of attaining the end. 

 When we use Natural Selection as the agent of this change, 

 we of course mean that the primary organic structure (in this 

 case the mesodermal cell rudiment arising diffusely from all 

 points of the ectoderm) was forced to respond to a further com- 

 bination of forces in the environment which we cannot define 

 in more exact physical terms. 



From this point of view the usual method of forming the 

 mesoderm in insects, by a well-marked gastrula groove, is not 

 an independent or accidental phenomenon, but has been derived 

 from a more primitive method of migration already established 

 in the earlier insects, not as a direct and necessary result of 

 apparent and readily stated mechanical conditions, but as a 

 response to additional forces, compelling an important change 

 in the older but less direct process which is still efficient in 

 some primitive insects. These "additional forces" (mechani- 

 cal, chemical, or what not), included under the general term 

 "adaptive," did not "necessarily" disturb in the Termite the 

 primitive habit established in their ancestors. In other insects, 

 when new conditions (mechanical or others) made it possible 

 and more desirable, invagination arose as a response. 



A study of the origin of the "lower-layer" in the Termite 

 shows a very close connection between this and the establish- 

 ment of the first rudiment of the embryo by a concentration of 

 the blastoderm cells toward a certain area (as in the case of 

 Isopods discovered by McMurrich). This more general phe- 

 nomenon must be first explained before attempting the special 

 problem of the exact mechanical nature of the origin of the 

 mesoderm, which is too intimately bound up with the solution 

 of the former question to be considered alone. 



As to the entoderm of the Termite, I must say that it 



