The Life of the Individual 



97 



second day, a thickening appears on the convex side, and, 

 on the anterior end (larger end) of the egg, the first indica- 

 tions of the future appendages are soon visible (Fig. 51, a 

 and b). These consist of the rudiments of the antennae 

 (Ant) and mouth parts (mandible, Md and maxillae, IMx, 

 2 MX) on the head and of the three pairs of legs (1L, 2L, 8L) 

 on the thorax. These rudiments are simply slightly rounded 

 swellings which are at first smaller toward the posterior 

 end of the egg, since development progresses from the an- 

 terior end. The embryo shows at first no division into 



ShGl 



FIG. 51. Three stages in the development of the embryo. 



head, thorax and abdomen, but the fate of the various 

 swellings must be determined by following them through. 

 The rudiments of the stigmata (Sp t openings of the tracheal 

 system) appear early and the first evidence of the silk glands 

 (SlkGl) becomes visible about the same time just behind the 

 second maxillae. The first external indication of the nerv- 

 ous system is in two pairs of swellings (Br) on the upper 

 side of the head. Even in this early stage, a number of 

 important organs are already outlined. 



