14 ANA TOMY AND MORPHOLOGY OF INSECTS. 



Its closed end comes into contact with, and is afterwards in- 

 vaginated in, the anterior extremity of the mesenteron, as the 

 archenteron is designated after the closure of the blastopore. 



Similar changes also occur in the proctodaeum. The par- 

 tition walls between the three sections of the alimentary canal 

 are subsequently absorbed. 



The primitive band becomes more marked as development 

 progresses, extends forwards at the lateral margins of the 

 orifice of the stomodaeum, and expands in front of this orifice 

 into a pair of broad lobes, which form the sides of the head — 

 the procephalic lobes. 



The primitive band is now seen to be divided by fine trans- 

 verse lines into a number of metameres or somites. This 

 division commences immediately behind the stomodaeum. 

 Three somites are first formed ; others succeed behind these 

 until the whole length of the primitive band is segmented. 

 Two lateral buds now appear, one on each side, on several of 

 the anterior somites, commencing on the somite immediately 

 behind the mouth, and appearing in order from before back- 

 wards. These are the rudiments of the ventral appendages — 

 the jaws and legs. It will be observed that there is no 

 segmentation of the parts in front of the stomodaeum, the 

 procephalic lobes and their pedicles. As in vertebrates, all the 

 primitive somites are post-oral. 



The appendages of the first three somites always become 

 the mouth-organs, and form a pair of mandibles, a pair of 

 maxill?e, and a labium or low^er lip, which is formed by the 

 more or less complete union of the third pair of lateral 

 appendages. 



The nature of the labium is very manifest in the Orthoptera, 

 Coleoptera, and Hymenoptera, even in the perfect insect. 



The three succeeding pairs of ventral appendages become 

 the thoracic limbs (legs). The number of post-oral metameres 

 which enter into the composition of the head is invariably 

 three, and the same number belong to the thorax ; all the 

 remainder are abdominal. There are probably rarely less 

 than nine, or more than eleven, of these in the embryo. The 



