No. 3-] THE EMBRYOLOGY OF A TERMITE. 54 1 



question whether the origin of organic structures is ultimately 

 purely a problem of mechanics, as a first cause, is not raised. 

 Here the contention is that certain evident and simply stated 

 conditions of pressure and mechanical strain are alone sufficient 

 to force the amnio-serosal fold to arise. 



Wheeler (26) advocates this idea concisely, as follows : "The 

 amnio-serosal fold is a mechanical result of a local induplica- 

 tion of the blastoderm, due to rapid proliferation in a single 

 layer of cells." " There is the vesicular one-layered blasto- 

 derm 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 invagi- 

 nation is favored by the displacement of yolk during its lique- 

 faction and absorption by the growing embryo. We may 

 suppose that this invagination, which results in the formation 

 of the amnio-serosal fold, assumed a definite and specific char- 

 acter in different groups of insects." 



Similar mechanical conditions are appealed to as the cause 

 of certain invaginations in other forms ; the Cestode head in 

 Cysticercus ; the Nemertine in the Pilidium ; the formation of 

 the young Spatangid in the Pluteus ; the development of the 

 amnion and serosa in vertebrates ; and the imaginal discs of 

 insects. 



a. I. Even if we admit the presence of just such a com- 

 bination of forces as is enumerated above, they seem to be 

 subsidiary and insufficient alone, without a further cause, to 

 explain the origin of the membranes for the following general 

 reasons : 



It must be recalled that no amnion results in the similar 

 rapidly proliferating areas of crustacean eggs, that such a 

 membrane is lacking among the myriopods and apterygote 

 insects (in spite of Heymons's (15) claim, which requires fur- 

 ther and more convincing proof, as we shall see later), and 

 that it is not formed in certain of the higher insects. It 

 should also be remembered that similar membranes are want- 



