DEVELOPMENT 



55 



Entoderm. At its anterior and posterior ends, the inner 

 layer just referred to gives rise to a mass of cells which are 



FIG. 20 1. 



/-my 



FIG. 202. 



ms 



Transverse section of abdomen of Clytra embryo at an advanced stage of develop- 

 ment, a, appendage; e, epithelium of mid intestine; g, ganglion; m, Malpighian tube; 

 mi, muscular layer of mid intestine; ms, muscle elements; my, mesenchyme (source 

 of fat -body) ; s, sexual organ; t, tracheal invagination. After LECAILLON. 



destined to form the mesenteron, from which the mid intestine 



develops. One mass is adjacent to the blind end pf the stomo- 



dseal invagination and the other to that of 



the proctodseal in-folding. The two 



masses become U-shaped (Fig. 202), and 



the lateral arms of the two elongate and 



join so that the entodermal masses become 



connected by two lateral strands of cells ; 



by overgrowth and undergrowth from 



these lateral strands a tube is formed 



which is destined to become the stomach, 



and by the disappearance of the partitions 



that separate the mesenteron from the 



stomodseum at one end and from the proc- 



todaeum at the other end, the continuity 



of the alimentary canal is established. 



The fore and the hind gut, then, are 



ectodermal in origin, and the mid gut 



entodermal. 



Diagram of formation 

 of entoderm in Leptino- 

 tarsa. e, e, entodermal 

 masses; in, mesoderm. 

 After WHEELER. 



