452 SUMMARY. 



The mesoblast in the Tracheata is formed in connection with, a 

 median thickening of the ventral plate. The unpaired plate of 

 mesoblast so formed becomes divided into two bauds, one on each 

 side of the middle line. 



In both Spiders and Myriopods, and probably Insects, the two 

 plates of mesoblast are subsequently divided into somites, the lumen 

 of which is continued into the limbs. 



In Crustacea the mesoblast usually originates from the walls of 

 the invagination, which gives rise to the mesenteron. 



It does not become divided into two distinct bands, but forms a layer 

 of scattered cells between the epiblast and hypoblast, and does not 

 usually break up into somites; and though somites are stated in some 

 cases to be found they do not resemble those in the Tracheata. 



The proctodseum is usually formed in Crustacea before and rarely 

 later 1 than the stomodseum. The reverse is true for the Tracheata. 

 In Crustacea the proctodaeum and stomodaeum, especially the former, 

 are very long, and usually give rise to the greater part of the alimen- 

 tary tract, while the mesenteron is usually short. 



In the Tracheata the mesenteron is always considerable, and the 

 proctodaaum is always short. The derivation of the Malpighian 

 bodies from the proctodseum is common to most Tracheata. Such 

 diverticula of the proctodaeum are not found in Crustacea. 



1 This is stated to be the case in Moina (Grobben). 



