Nos. 455-456.] 



HEAD OF BLATTA. 



ance shown in Figure 8. They have not yet fused, but the 

 future labrum is clearly outlined. In the meantime they have 

 also migrated forward, so that the space which formerly sepa- 

 rated them from the ist maxillas has been considerably reduced, 

 and in lateral view the 2nd maxillae are more or less hidden by 

 the first pair. 



About this time there appears a prominent imagination lying 

 cephalo-mesad of the base of the mandible, close to the mandib- 

 ular ganglion. This lies under the antenna-, but can W scon 

 by focusing below that appendage or by rcmo\ in- it d 'l,-. 9, 

 a.t.). In the same embryos there are iironiinent ten juiis of 

 tracheal invaginations on the plura; of ^ . . . • . 



the meso-, and metathorax and tlie 

 first eight abdominal segments. For 

 the prothorax there is a pair of slight 

 and indistinct invaginations which have 

 escaped the attention of previous work- 

 ers on Blatta. It should be noted that 

 the location of the above mentioned 

 mandibular invaginations is in no wise 

 comparable to that of the tracheae. In 

 addition to these invaginations there are 



in the mouth-part region three other i : :..„ 

 pairs — one pair at the cephalo-lateral n .'.mtc- 



angle of the mandibles, one between the rioi tcumnai iMv.mHi,.ii..n wr. /.i 

 mandibles and the maxillae, close to the 



caudo-lateral angle of the mandibles, and one cephalo-laterad 

 of the base of the 2nd maxillae. The last i)air of in\ :i-iiKiti< ms 

 is difficult to detect, for they are smaller, their lumen is not 

 prominent and is frequently hidden by the caudal margin ot the 

 1st maxilke. It is shown in Figure 10,/./. 'J'hc in\a_ginati(.ns 

 lying laterad of the mandibles form tendons tor the mandibular 

 niuscles. The role of the first and the last i)airs in the forma- 

 tion of the tentorium or endoskeleton of the head will be dis- 

 cussed later. 



Up to this jjcriod the length of the mouth-part re-ion has 

 considerably exceeded that of the combined ocular and antennal 

 segments. Thus, as we have seen, in an eml)ryo of about eleven 



