404 



ANNUAL EEPOKT SMITHSONTAN INSTITUTION, 1925 



AMR 



Stom. 

 AMR 



narrow sutures (fig. 19 B, &), but at other places, as between the 

 body segments, the soft " membranous" areas are infolded (a), pro- 

 ducing a movable joint. In many cases the hard parts of the skele- 

 ton are articulated by ball-and-socket joints, as at the bases of the 



jaws and between the seg- 

 ments of the legs, but an ar- 

 ticulation of this sort in an 

 insect is merely one between 

 two extensions of the hard 

 cuticula from the opposing 

 edges of the neighboring 

 plates (fig. 19 C) on the 

 outside of the continuous 

 though membranous part of 

 the body wall. The articu- 

 lations in the skeleton of in- 

 sects are thus of quite a dif- 

 ferent nature from tlic joints 

 of a vertebrate skeleton. 



The various kinds of 

 hairs, spines, and scales that 

 may cover the body and 

 appendages of an insect are 

 outgrowtlis of the cuticula. 

 Internal ridges and arms 

 that brace the skeleton or 

 furnish attachment points 

 for muscles are ingrowths 

 of the cuticula within in- 

 foldings of the hypodermis. 

 All the sense organs of in- 

 sects ai-e products of the 

 ectoderm, and consist of 

 both cuticular and hypoder- 

 mal parts, the sensory cells 

 being specialized hypoder- 

 mal cells with nerve con- 

 nections. 



MtK 



Fig. 20. — Formation of the alimentary caual 

 (diagrammatic lengthwise sections) 

 A, the ventral blastoderm differentiated into 

 ectoderm (Ect), mesoderm {Meso), and a mass 

 of cells growing inward at each end, the an- 

 terior and posterior mesenteron rndiments 

 (AMR and PMR). B, mesoderm disappearing 

 along midline ; mescntpron rudiments extending 

 as cups around the yolk (Y) ; an ectodermal In- 

 growth taking place at each end of embryo, the 

 stomodeum (Stom) in front, the proctodeum 

 (l'roc\ belilnd. C, stomodeum and proctodeum 

 deeper; lips of mesenteron rudiments approach- 

 ing eiich other. D, mesenteron rudiments united 

 about yolk to form the stomach, or vontrlculus 

 (Vent) ; stomodeum and proctodeum opened into 

 stomach, the first becoming the oesopliagus 

 (G<!) and associated parts of alimentary canal 

 (fig. 21) opening from mouth (Mth), the second 

 the intestine (Int) opening to exterior through 

 the anus (An). 



THE ALIMENTARY CANAL 



To follow the development of the embryo in all its parts at the 

 same time would be something like tiying to follow the plot in one 

 of those old-fashioned novels that drags along through the lives of 

 a dozen or more characters and only lets you Imow what they all 



