154 THE BIOLOGY OF INSECTS 



backwards between these two, so that the tail is pushed 

 farther and farther from the head until the whole series of 

 head, thoracic, and abdominal segments have been formed ; 

 and on some of them the rudiments of limbs bud out. This 

 body-segmentation with the series of appendages can 

 be observed on the surface of the developing embryo 



(Fig- 40)- 



But deeper investigation by examination of serial sections 



shows the origin and elaboration of various sets of organs. 

 The outer layer (ectoderm) gives rise not only to the skin 

 (epidermis) but, as in animals generally, to the nervous 

 system, whose rudiments sink in as a pair of elongated 

 segmented ridges whence are formed the series of ganglia 

 with their connecting cords. Fine paired inpushings of 

 ectoderm on most of the segments mark the position of 

 the spiracles and furnish the rudiments of the tracheal or 

 air-tube system. Median inpushings of the ectoderm near 

 the front end of the embryo and at the tail indicate the 

 future mouth and vent, and grow into the fore-gut and 

 hind-gut respectively ; these, it will be remembered (pp. 

 5, 23-8, 30-1), as well as the air-tubes, are lined with cuticle 

 in the developed insect. As growth proceeds, the mouth, 

 originally in front of or between the feelers, moves back- 

 wards so that it comes to lie between and behind the 

 mandibles. 



The lower layer is necessarily situated between the 

 ectodermic structures just mentioned and the yolk ; most 

 of it grows to form a series of segmental cell-masses (meso- 

 dermal somites) each with a pair of cavities (coelomic spaces). 

 From these cell-masses the muscles and connective tissues 

 of the body are formed, regions of many of them growing 

 out into the developing limbs as they arise. Near the walls 

 of certain of the coelomic spaces the primitive germ-cells 

 appear, and these spaces themselves become the cavities of 

 the reproductive organs and their ducts (ovaries and oviducts 

 in the female, testes and vasa deferentia in the male). The 

 mesoderm grows dorsalwards on either side beneath the 

 ectoderm, forming masses of loose tissue, from this the 



