414 INSECTA. 



Ventrally the mesoblastic bands soon meet across the median line. The 

 cavities in the appendages become obliterated and their mesoblastic 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 aliment- 

 ary tract. The mesoblastic wall of the proctodaeum 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 antennas and upper 

 lip. 



The alimentary canal. The alimentary tract of Insects is 

 formed of three distinct sections (fig. 181) a mesenteron or 

 middle section (me), a stomodaeum (st) and a proctodaeum (an}. 

 The stomodaeum and proctodaeum are invaginations 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 stomo- 

 daeum 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 

 proctodaeum, which includes the whole of the region from their 

 insertion to the anus. 



The oral invagination appears nearly coincidently 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 outgroivths of the 

 epiblast of the proctod&um, 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. Tichomiroff (No. 420) confirms Kowalevsky on this point, 



