SUMMARY. 543 



evidence is in favour of such an invagination being the usual, 

 and, without doubt, the primitive, mode of origin. 



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 bands, 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 hypo- 

 blast, 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 stomodaeum. 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 alimentary tract, while the mesenteron is 

 usually short. 



In the Tracheata the mesenteron is always considerable, and 

 the proctodaeum is always short. The derivation of the Mal- 

 pighian 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 (Grobhen). 



