Nos. 455-456.] 



HEAD OF BLATTA. 



do not believe that it can be regarded as absolute. At any rate, 

 the evidence against the labrum as the sternite of the third seg- 

 ment completely outweighs this consideration.^ 



The procephalon develops entirely in front of the stomodaeal 

 invagination, and is already well outlined before the deutocere- 

 bral and tritocerebral ganglia have moved forward. Moreover, 

 as Prof. Comstock suggested, the labrum and the clypeus give 

 no satisfactory evidence of a paired origin. The absence of 

 median sutures in these sclerites might be readily explained by 

 the statement that fusion had taken place at such an early stage 

 as to be complete. But a more serious objection is that if they 

 develop post-orally and migrate to their pre-oral position, there 

 should be a stage in which they are paired. Such a stage does 



In the Orthoptera and in the Pterygota generally, the second 

 antennal segment is so slightly developed and so transient as to 

 have been largely overlooked by investigators, yet in these forms 

 the labrum is usually strongly developed. On the other hand, 

 in the Apterygota, where the second antennal segment is 

 especially well developed, the labrum is comparatively insig- 

 nificant. 



Finally, the sutures separating the front, clypeus, and labrum 

 are developed at a comparatively late period in the development 

 of the procephalon. If these sclerites represented primary seg- 

 ments, we should find them developing from three distinct cen- 

 ters. The fact that the procephalon develops as a single piece 

 shows that the sclerites derived from it are not primary, but are 

 secondarily developed. In fact, they are not constant, but there 

 are many species in which one or more of the sutures is lacking, 

 and thus the number of sclerites reduced. 



The vertex, the compound eyes, and with them the ocular 

 sclerites and the gen^e, are formed from the fused cephalic lobes 

 and thus, with the front, clypeus and labrum, belong to^ the 

 ocular or protocerebral segment. The 



epicranium represents the ventral 



these lobes. 



