TRACE EAT A. 



331 



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. 335. The heart is 

 formed from the mesoblastic layers, 

 where they meet in the middle dorsal 

 line (fig. 185 C, hf). The somatic me- 

 soblast 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 thehypoblast in its growth 

 round the yolk. The proctodseum forms 

 the rectum and JVIalpighian bodies 1 , 

 and the stomodseum 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 gene- 

 rally, but it is necessary to follow with 

 somewhat greater detail the compara- 

 tive 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, which usually 

 contains nuclei, and in the Poduridse is divided up in the ordinary 

 segmentation into distinct yolk cells. The first definite structure 

 formed is a thickening of the blastoderm, which forms a ventral 

 plate. 



The ventral plate is very differently situated in relation to the yolk in 

 different types. In most Diptera, Hymenoptera and (?) Neuroptera (Phry- 

 ganea) it forms from the first a thickening extending over nearly the 



FlG. 180. TWO STAGES IN THE 

 DEVELOPMENT OF HYDROPHILUS PI- 

 CE US. (From Gegenbaur, after 

 Kowalevsky. ) 



Is. labruin; at. antenna; md. 

 mandible; mx. maxilla I.; U. max- 

 illa H. ; p'p"p'". feet ; a. anus. 



1 This lias not been shewn in the case of Hydrophilus. 



