544 KNOWER. [Vol. XVI. 



If the Ryder- Wheeler mechanical theory were correct, the 

 most natural place to expect the fold would be just at the junc- 

 tion of the rapidly proliferating germ-disc with what is claimed 

 as a rigid, resistant, extra-embryonal region. We would look 

 for the weakest point here. The fold does not, however, occur 

 here in the Termite or other amniote insects. 



My own observations, and a general review of the question, 

 lead me to believe that the embryonic membranes of insects 

 are adaptive structures, which arose in the winged insects as a 

 response to some definite need of the embryo. I do not think 

 the exact combination of physico-chemical forces, cooperating 

 to bring about this result, can be stated at present. The eggs 

 of the anamniote apterygota are, to all appearances, as far as 

 mechanical conditions go, similar to those of winged insects. 

 The physical constitution of the ^%g was already favorable 

 to the origin of the amnion in the ancestors of the latter 

 forms ; but before one arose, certain additional forces were 

 necessary, which must be associated with some necessity of 

 covering over the embryo at an early stage. Whether this 

 necessity (physico-chemical, no doubt) was one of protection, 

 prevention of evaporation, better nutrition, or to furnish a 

 depository for waste products, may not be decided ; but any 

 one of these suggestions, or all together, would be reasonable 

 cause. 



When forms arose among the higher insects, as adaptations 

 to special new conditions, the early completion of the process 

 became less important ; and in a few cases the amnion ceased 

 to appear, being no longer needed. (If it is any more precise, 

 we may say that the amnion was no longer maintained by the 

 physico-chemical forces which originated it.) 



B. I must refer to Wagner's (23) comprehensive theory of 

 the origin of insect embryonic membranes and other organic 

 structures, as another example of a simple, clear-cut mechanical 

 explanation of such problems, which also illustrates the diffi- 

 culty of correctly estimating and balancing forces, and their 

 necessary effect on organized matter. 



In a few words, his idea is as follows : Think of the similar 

 cells of a uniform epithelium as an organic molecule, so built 



