540 KNOVVER. [Vol. XVI. 



appears late, after the segmentation of the germ-band. The 

 yolk-cells (as both Heymons and Wheeler claim) can take no 

 part in the formation of this layer; since at an early stage, 

 before the closure of the amniotic cavity, they have become 

 very large and unlike the cells which later form the entoderm. 



The fact that this layer arises so constantly among insects 

 with the mesoderm at the two ends of the invagination, termed 

 "gastrula" (see Wheeler (26)), is a strong point against Hey- 

 mons's assumption of the independent, accidental character of 

 this groove. 



I shall be obliged to defer to another time the discussion of 

 the method of the origin of the entoderm, its exact relation 

 to the mesoderm and to the gastrula groove, when this occurs, 

 as well as its association with the stomodeal and proctodeal 

 invaginations. 



The Origin of the Amnion in Insects. 



The discussion as to the cause of, and the primitive method 

 of origin of, the embryonic membranes of insects has at least 

 developed some extremely interesting ideas. 



At present, opinions seem to halt between, first, the Ryder- 

 Wheeler (26) hypothesis of a purely mechanical and independ- 

 ent origin of the amnio-serosal fold among the winged insects ; 

 and, second, the theory of Will (27), Wheeler (25), and Korschelt 

 and Heider (17), recently championed by Heymons (15), which 

 associates the formation of embryonic membranes in insects, 

 more or less closely, with a certain phenomenon exhibited 

 by the myriopod embryo. Wagner's (23) views I shall put, for 

 convenience, in the first category ; while Willey's (29) recent 

 contribution, though in some respects agreeing with the sec- 

 ond, will have to be considered alone. 



A. Examining first the Ryder- Wheeler theory, we find that 

 Wheeler (26) has adapted Ryder's (22) "mechanical explana- 

 tion" for the origin of the amnion of vertebrates to the insect 

 amnion. Of course the term " mechanical " is here used in its 

 narrower sense, referring the question to immediate antecedent 

 causes, which alone are claimed to necessitate the result. The 



