62 WHEELER. [Vol. VIII. 



(3) the chitinous cuticles. As stated above, the amnions of 

 Peripatiis and Scorpions probably also represent structures 

 of independent origin and no wise homologous with the 

 envelopes of insects. It is perhaps unnecessary to add that 

 the reduction of all these structures to the annelid trochophore 

 is in the present state of our knowledge little more than a wild 



guess. 



Graber ('90) has criticized the view advanced by Will and 

 myself, that the insect amnion arose by an invagination of the 

 germ-band like that of some Myriopods {Gcophilns). His 

 contention is certainly in great measure well-founded. Still 

 I believe that it does not affect the essential point of the 

 hypothesis which implies that the amnioserosal fold is the 

 mechanical result of a local induplication of the blastoderm 

 due to rapid proliferation in a single layer of cells. 



Ryder ('86) has sought a mechanical explanation for the 

 amnion, and although his paper treats mainly of the vertebrate 

 amnion, he evidently implies that the homonymous envelope of 

 the Insecta had a similar origin. According to him "the 

 amnion in all forms has arisen in consequence of the forces 

 of growth resident in the embryo, encountering peripheral 

 and external resistance either in the form of a rigid outer 

 egg-shell (zona radiata) or decidua reflexa, or even the walls 

 of the uterine cavity itself, supposing of course that a large 

 vesicular blastoderm containing yolk has been formed by 

 epiboly." 



This view applies with little alteration to the Insecta. There 

 is the vesicular one-layered blastoderm filled with yolk and the 

 germ-band arising by rapid proliferation at one point. The 

 resistance of the yolk being less than the external resistance 

 of the tightly fitting chorion and vitelline membrane on the 

 one hand combined with the peripheral resistance of the extra- 

 embryonal blastoderm on the other, the germ-band is forced to 

 invaginate. This invaginative process is favored by the dis- 

 placement of yolk during its liquefaction and absorption by the 

 growing embryo. We may suppose that this invagination 

 which results in the formation of the amnioserosal fold, 

 assumed a definite and specific character in different groups of 

 insects. 



