96 The Life-History of Insects 



which are overgrown by the outer skin, ultimately 

 modified into the lens. The nerve-centres and cords 

 which lie above the gullet and crop arise as an out- 

 growth from the wall of the fore-gut (53). 



Fore and Hind-gut. — As mentioned above, the 

 fore and hind guts, which give rise to the greater 

 part of the food-canal, are inpushings of the ectoderm. 

 The fore-gut {stomodmim) usually appears earlier than 

 the hind-gut { proctodauni). The kidney-tubes arise 

 as outgrowths from the latter. The spittle-glands 

 are developed from inpushings of the ectoderm of the 

 head region, similar to the rudiments of the air-tubes. 

 As growth proceeds, these inpushings, originally 

 opening to the outer surface of the blastoderm, 

 become drawn into the mouth-cavity (2, 53). 



Dorsal Closure. — It is evident that the primitive 

 streak with the nerve-cord developing along its central 

 line, and the limbs arising from its outer surface, 

 marks the lower or ventral aspect of the future 

 insect. At its first formation it occupies but a small 

 part of the outer area of the egg, but as growth 

 proceeds it elongates and widens. That the body of 

 the future insect may be completed, the back or dorsal 

 aspect must be formed by the extension of the 

 embryo so as to completely enclose the yolk. This 

 process differs considerably in various insects. In 

 some cases (Dragon-flies) there is between the amnion 

 and the serosa behind the head an inpushing of 

 thickened cells, which ultimately form the dorsal 

 body-wall. Or the amnion may, as in some Beetles, 

 become ruptured and turned back from covering the 

 primitive streak until it encloses the yolk dorsally, 

 being finally absorbed as the primitive streak itself 

 spreads to form the dorsal wall. Or, as in some 

 Midges and Caddis-flies, a similar operation may be 

 carried out with the serosa. Or, as in Moths and 



