DEVELOPMENT 125 



mesodermal tissues. From the ccelom sacs develop the muscles, fat- 

 body, dorsal vessel, blood corpuscles, ovaries and testes; the external 

 sexual organs, however, as well as the vagina and ejaculatory duct, are 

 ectodermal in origin. 



Entoderm. At its anterior and posterior ends, the inner layer just 

 referred to gives rise to a mass of cells which are destined to form the 

 mesenteron, from which the mid intestine develops. One mass is ad- 

 jacent to the blind end of the stomodasal invagination and the other to 

 that of the proctodaeal in-folding. The two masses become U-shaped 

 (Fig. 203), and the lateral arms of the two elongate and join so that the 

 entodermal masses become connected by two lateral strands of cells; 



-7- -5 



-/--my 



m s 



FIG. 202. Transverse section of abdomen of Clytra embryo at 

 an advanced stage of development, 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; /, tracheal invagination. After LECAIL- 

 LON. 



FIG. 203. Dia- 

 gram of formation 

 of entoderm in Lep- 

 tinotarsa. e, e, en- 

 todermal masses; m. 

 mesoderm. After 

 WHEELER. 



by overgrowth and undergrowth from these lateral strands a tube is 

 formed which is destined to become the stomach, and by the disappear- 

 ance of the partitions that separate the mesenteron from the stomodaeum 

 at one end and from the proctodaeum 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. 



Polyembryony. In certain Hymenoptera a single egg may give 

 rise to many individuals. Thus in some Chalcididae and Proctotrypidae, 

 according to Marchal, the fertilized ovum segments into many (12-100) 

 embryos, which develop into as many adults, all the individuals from the 

 same ovum being of the same sex. 



