DEVELOPMENT. 



645 



adult, and as a rule the development of the somites and their 

 appendages takes place from before backwards. An anterior 

 ectoderm invagination to form the stomodaeum and a similar 

 insinking posteriorly to form the proctodaeum occur about the 

 same time as the primitive limbs begin to project. The second 

 of these limbs, the antennae, are at first postoral but soon shift 

 in front of the mouth. In many developing insects rudiments of 

 abdominal appendages have been seen, but these all disappear 

 at an early age, excepting in the Collembola and Thysanura. 

 The nervous sys- 

 tem arises as an 

 ectodermal thick- 

 ening before the 

 limbs appear and 

 quickly undergoes 

 a segmenta t i o n 

 into ganglia. The 

 tracheae arise as 

 ectod e r m a 1 i n- 

 vaginations, typi- 

 cally a pair in each 

 segment of the 

 body. 



Whilst these 

 various processes, 

 more or less ex- 

 ternally visible, 

 have been going 

 on, the embry- 

 onic band has been 

 growing all along 



its sides and thus gradually ensheathmg the yolk. The larger- 

 part of the layer of cells which we have referred to above as 

 the '' lower-layer," becomes mesoderm, but anteriorly around 

 the base of the stomodaeum and posteriorly around the base 

 of the proctodaeum some of the cells of the "lower-layer" 

 form the rudiments of the endoderm. These rudiments grow to- 

 wards each other, forming two V-shaped bands which ultimately 

 fuse and then broaden out, enclosing between them a large part 

 of the food-yolk. The two bands first meet in the ventral line 



ce 



Fin. 405. Embryos of Hydrophilus with limb-rudiments (after 

 Heicler, from Laiig's Tent-book), a aiial aperture ; an antenna ; 

 g rudiment of the ventral chain of ganglia ; TO oral apetture ; 



md mandible ; 



first, mx? second maxilla ; pi, p 2 , p 3 the 



tliiee pairs of thoracic limbs ; p 4 , p r \ p 7 , j; 9 rudiments of the 

 first six abdominal limbs ; st stigmata ; i'k procephalon. 



