552 



KNOWER. [Vol. XVI. 



There is, however, undoubtedly a resemblance between this 

 invagination and the phenomenon exhibited by the myriopod 

 embryo, which is strengthened by the appearance of the same 

 condition in the apterygote egg. This suggests strongly a 

 common cause (the general adaptive nature of which I have 

 suggested in agreement with others already quoted) ; but 

 Korschelt and Heider's ((17), pp. 734 and 787) further idea, 

 that this cause is associated necessarily with the resistance 

 offered by the spherical chorion to the growth of the elongat- 

 ing germ-band, does not seem convincing, since these authors 

 themselves suggest an objection in the different behavior of 

 some myriopods, see (17), p. 735 ; since similar conditions do 

 not necessitate a like invagination in certain insects, or in the 

 elongating band of the arachnid or in that of a fish, for reasons 

 I have suggested in another place ; and since the amniotic 

 fold of the Termite arises on the nearly circular disc, before 

 such conditions would be effective, 



e. 8. The possibility of a connection with the invagination 

 (<«doubling-up ") of the myriopod embryo seems sufficiently 

 strong to warrant a new statement of how a fundamentally simi- 

 lar invagination, in the primary embryonic rudiment of the 

 myriopod-like ancestors of winged insects, may have formed a 

 starting point for the formation of an amnion. 



As has been pointed out, some theory associating the two 

 invaginations has seemed probable to a number of investigators. 



Will (27), basing his theory on a study of the Hemipteran 

 embryo, first insisted on a derivation of the amnion from a 

 region of the myriopod body. 



Wheeler (25), at about the same time and independently of 

 Will, advocated much the same idea, though he simply quotes 

 Will in regard to the degeneration of segments into an amnion. 



Graber (9) justly criticised the idea of a disappearance of 

 certain posterior abdominal segments of the myriopod-like 

 ancestors of the insects, by a degeneration into an amnion and 

 a forward migration of the anus. 



Finally Heider (13), and later Korschelt and Heider (17), 

 presented a modification of the theory I have outlined, which. 



