400 INSECTA. 



(fig. 189 C). The two nervous strands are continuous in front 

 with the supra-cesophageal ganglia, which are formed of the 

 epiblast of the procephalic lobes. These plates gradually grow 

 round the dorsal side of the embryo, and there is formed 

 immediately behind them an oral invagination, in front of which 

 there appears an upper lip (fig. 180, Is). A proctodaeum is formed 

 at the hind end of the body slightly later than the stomodaeum. 

 The mesoblast cells become divided into two bands, one on 

 each side of the middle line (fig. 189 A), and split into 

 splanchnic and somatic layers. The central yolk mass at about 

 the stage represented in fig. 179 begins to break up into 

 yolk spheres. The hypoblast is formed first on the ventral 

 side at the junction of the mesoblast and the yolk, and 

 gradually extends and forms a complete sack-like mesenteron, 

 enveloping the yolk (fig. 185 al]. The amnion and serous 

 membrane retain their primitive constitution for some time, but 

 gradually become thinner on the ventral surface, where a rupture 

 appears eventually to take place. The greater part of them 

 disappears, but in the closure of the dorsal parietes the serous 

 envelope plays a peculiar part, which is not yet understood. It 

 is described on p. 404. The heart is formed from the mesoblas- 

 tic layers, where they meet in the middle dorsal line (fig. 185 C, 

 ht\ The somatic mesoblast gives rise to the muscles and 

 connective tissue, and the splanchnic mesoblast to the muscular 

 part of the wall of the alimentary tract, which accompanies the 

 hypoblast in its growth round the yolk. The proctodaeum 

 forms the rectum and Malpighian bodies 1 , and the stomodaeum 

 the oesophagus and proventriculus. The-two epiblastic sections 

 of the alimentary tract are eventually placed in communication 

 with the mesenteron. 



The development of Hydrophilus is a fair type of that of 

 Insects generally, but it is necessary to follow with somewhat 

 greater detail the comparative history of the various parts which 

 have been briefly described for this type. 



The embryonic membranes and the formation of the layers. 



All Insects have at the close of segmentation a blastoderm 

 formed of a single row of cells enclosing a central yolk mass, 



1 This has not been shewn in the case of Hydrophilus, 



