32 INSECT TRANSFORMATION 



as various groups of cells become more and more differentiated 

 for special purposes. In the development of most insects, the 

 thin blastoderm surrounding the germ-band, grows in such a 

 way as to form a protective sheet or amnion over the latter 

 in which the main features of the body-form are progressively 

 marked out (Fig. 17). The segmentation of the body is indi- 

 cated by transverse furrows across the germ-band. At a very 

 early stage, primitive head- and tail-regions may be distin- 

 guished, and the series of limb-bearing segments are formed in 

 regular order from before backwards, so that the tail-region 

 is pushed farther and farther from the head as growth proceeds. 

 In each segment there may be present a pair of nerve-ganglia, 

 and a pair of cavities (codomic spaces) in the mesoderm tissue 

 derived from the inner cell-layer, while on each segment a pair 

 of limbs or appendages may grow out. The last-named struc- 

 tures may, as the legs of the embryo grasshopper, cockroach, 

 or bug, grow so rapidly that by the time of hatching they 

 resemble closely those of the adult ; or they may remain in a 

 rudimentary condition, as for example do the thoracic legs of 

 certain beetle-grubs ; or they may, after having attained slight 

 development disappear entirely before hatching, as is the case 

 with the rudimentary limbs on the anterior abdominal seg- 

 ments of other beetle embryos. 



The inpushings of the ectoderm to form the front- and hind- 

 regions of the food-canal appear at an early stage in the mid- 

 ventral line of the germ-band, the opening of the fore-gut the 

 future mouth just behind the primitive head, and that of the 

 hind-gut on the primitive tail-region. As growth proceeds the 

 mouth moves backwards from its original position in front of 

 the feelers to its final place behind the mandibles and between 

 the maxillae. The fore-gut and hind-gut grow inwards to 

 communicate with the insect's central digestive cavity in which 

 the store of food-yolk, enclosed by the growth of the embryo, 

 is gradually absorbed. 



The close of embryonic development leads on to the impor- 

 tant operation of hatching which ushers the young insect into 

 its free, active life. The details of the hatching process vary 

 greatly among insects of different groups, and some of these 

 will be mentioned in subsequent pages of this volume. For the 

 present it may suffice to describe briefly the emergence of the 



