Nos. 455-456.] HEAD OF BLATTA. 



807 



My views in regard to the relations of the sclerites of the 

 adult Blatta to the primitive segments may be summed up in the 

 following table : 



BVom the above results it would seem that the definitive 

 sclerites can afford us little or no evidence as to the primary 

 segmentation of the insects. This is certainly true of the head 

 sclerites, and I see no reason why it should not apply to other 

 regions of the body. Sclerites originate from mechanical causes, 

 and do not necessarily have any relation to the primary segmen- 

 tation. 



As an illustration might be cited the breaking up of the 

 annular segment into sternite, pleurites and tergite. The cir- 

 cumcrescence of yolk and the consequent formation of the lateral 

 and dorsal portions of the embryo is a process of gradual growth, 

 and there is no demarcation into separate areas. The distinction 

 into sternite, pleurites, and tergite is a secondary process. 



More germane is the fact of the origin of the front, clypeus, 

 and labrum from the procephalon. Thus not only these three 

 sclerites but also the vertex and the gen^e arise from the one 

 segment, the protocerebral. I have shown, also, that the post- 

 genae are to be regarded, not as the pleurites of a single segment, 

 but as the fused pleurites of the mandibular and maxillary seg- 

 ments. 



I would not depreciate the value of the sclerites as an index 

 . to relationship among insects. That they are to a marked 



