Nos. 455-456.] 



HEAD OF BLATTA. 



795 



tion with the body-wall in the region of the antennal sclerites. 

 Figure 1 1 shows diagrammatically the course of the tentorial 

 arms in an embryo of about eighteen days. 



Considering the rapid growth of the embryo during the 

 period following their appearance, the development of these 

 rudiments is but slow. It is not until the twentieth day or 

 later that the fusion of the two pairs of rudiments has taken 

 place. 



In view of this method of origin of the dor.sal arms of the 

 tentorium of Blatta, it is interesting to note the condition in the 

 Plecoptera, a group in many respects more generalized than the 

 Blattida;. Of the dorsal arms in these forms Comstock and 

 Kochi, p. 41, say : " In the Plecoptera it appears to be merely 

 a chitinized tendon, the peripheral end of which is less c hitiuizcd 

 than the base and is only loosely attached to the skull. " Believ- 

 ing that in other insects the anterior arms arise from a distinct 

 invagination, these authors continue : It remains to be deter- 

 mined whether or not the dorsal arms in the Plecoptera are 

 homologous with the apodeme-like dorsal arms in other insects 

 and, if so, which type is the more generalized." As has been 

 seen, the evidence of embryology shows that the generalized con- 

 dition is that which still persists in the Plecoptera. 



The most thorough study of the development of the tento- 

 rium was that of Heider ('89). In addition to this investigator, 

 several others have devoted more or less attention to the sub- 

 ject. Hatschek ('77) called attention to what he supi^osed weiv 

 tracheal invaginations in the head of lei)idoi.terous einl)i\M^. 

 Tichomiroff ('79) says : -There exi.ts no hcid trache:. su. h ■ 

 Hatschek thought he discovered. The epithelial mva-inan-;. - 

 h^-re found I believe to be the rudiments of the inner head-ke!- 



\vas unable to clettrmine their nMe. He inclined to the belief 

 ^li^^t they disai)pear altogether, - as no trace of them wa> to be 

 f'Hind in sections of an embryo about the time of hatching. 

 Wheeler ('89) attributed the tentorium to five pairs of imagina- 

 tions anterior to the maxillarv segment. 



