408 



ANNUAL REPORT SMITHSONIAN INSTITUTION, 1925 



which serves as an outlet eaiial. or duct {Dct). Others are simple. 

 lioUow. tubular, or saclike ingrowths of the body wall (B), while 

 still others are of the same nature but are complicated in form by 

 bein<i- branched or by having secondary outgroAvths from the main 

 tube (C), the latter often being enlarged to form a reservoir {Res.). 

 Not much is known concerning internal ductless glands in in- 

 sects, but many species have certain large, free cells within the 

 bod}' cavit}'^, laiown as amocyfes, Avhich appear to be of this nature. 

 Most investigators claim that the oenocytes arise from the ectoderm 

 of the embryo along the sides of the bod3\ If they are glandular 

 in function, the}' are probably, then, ectodermal glands cells de- 



FiG. -3. — Three t.vpt's of insect glands derived from the ectoderm. 



A, a one-celled gland (fJICl) which is an enlarged cell of the hypo- 

 dormis (Hy), containing often a tubular ingrowth (Det) of the cutic- 

 ula (Vt) serving as an outlet duct. B, a simple many-celled gland 

 (GO formed as a hollow ingrowth of the body wall, in which the cutic- 

 ula becomes the lining intima (In), and the narrowed neck serves as a 

 duct (Dct). C, a compound many-celk-d gland produced by a branching 

 of type B, often with an enlarged part of the common duct (Dct) 

 serving as a reservoir (Res). 



tached from their original sites and become free, internal, one- 

 celled organs. In the head, just behind the brain, there is a pair 

 of small cellular bodies, Imown as the corpora aUata, likewise 

 deriA'ed from the ectoderm, and these, too, are often supposed to 

 be internal ductless grlands. 



THK MUSCLES 



The muscle tissue is derived from the mesoderm, though it later 

 becomes intimately attached to the ectoderm. The mesoderm is at 

 hrst usually a solid layer of cells (fig. 13, C, E, F, 3feso) in the 

 ventral part of the young embryo, continuous from one side to the 

 other, but as it grows upward in the sides it breaks apart in the 

 middle and leaves an empty space beneath the alimentary canal. 



