TRACE EAT A. 343 



walls form the muscles, etc. The cavities in the separate mesoblastic 

 somites also cease to be distinctly circumscribed. 



The splanchnic mesoblast follows the hypoblast in its growth, and gives 

 rise to the connective tissue and muscular parts of the walls of the ali- 

 mentary tract. The mesoblastic wall of the proctodseum is probably formed 

 independently of the mesoblastic somites. In the head the mesoblast is 

 stated to form at first a median ventral mass, which does not pass into the 

 procephalic lobe ; though it assists in forming both the antennae and upper 

 lip. 



The alimentary canal. The alimentary tract of Insects is formed 

 of three distinct sections (tig. 181) ameseuteron or middle section (me), 

 a stomodoaurn (st) and a proctodaeum (an). The stomodaeum and 

 proctodseum are imaginations of the epiblast, while the mesenteron is 

 lined by the hypoblast. The distinction between the three is usually 

 well marked in the adult by the epiblastic derivatives being lined 

 by chitin. The stomodaBum consists of mouth, oesophagus, crop, and 

 proventriculus or gizzard, when such are present. The mesenteron 

 includes the stomach, and is sometimes (Orthoptera, etc.) provided at 

 its front end with pyloric diverticula posteriorly it terminates just 

 in front of the Malpighian bodies. These latter fall into the procto- 

 dneum, which includes the whole of the region from their insertion to 

 the anus. 



The oral invagination appears nearly coiucidently with the first 

 formation of segments at the front end of the groove between the 

 lateral nerve cords, and the anal invagination appears slightly later 

 at the hindermost end of the ventral plate. 



The Malpighian bodies arise as two pairs of outgrowths of the 

 epiblast of the proctodcvuni, whether solid at first is not certain. The 

 subsequent increase which usually takes place in their number is due 

 to sproutings (at first solid) of the two original vessels. 



The glandular walls of the mesenteron are formed from the hypoblast ; 

 but the exact origin of the layer has not been thoroughly worked out in all 

 cases. In Hydrophilus it is stated by Kowalevsky (No. 416) to appear as 

 two sheets split off from the lateral masses of mesoblast, which gradually 

 grow round the yolk, and a similar mode of formation would seem to hold 

 good for Apis. Ticlioniiroff (No. 420) confirms Kowalevsky on this point, 

 and further states that these two masses meet first ventrally and much 

 later on the dorsal side. In Lepidoptera, on the other hand, Hatschek 

 finds that the hypoblast arises as a median mass of polygonal cells in the 

 anterior part of the ventral plate. These cells increase by absorbing material 

 from the yolk, and then gradually extend themselves and grow round 

 the yolk. 



Dohrn (No. 408) believes that the yolk cells, the origin of which has 

 already been spoken of, give rise to the hypoblastic walls of the mesenteron, 

 and this view appears to be shared by Graber (No. 412), though the latter 

 author holds that some of the yolk cells are derived by budding from the 

 blastoderm 1 . 



1 G-raber's view on this point may probably be explained by supposing that he has 



