556 KNOWER. [Vol. XVI. 



(ectodermic) and lower (mesodermic) layers when once estab- 

 lished. Each layer develops certain structures peculiar to it 

 (text-figures). The primitive ectoderm alone would, on a priori 

 grounds, be expected to be the layer to differentiate a protective 

 structure, as the amnion has been thought to be. Finally, in 

 the Termite the doubling-up to form the ectodermic amniotic 

 fold takes place distinctly before the mesoderm has spread 

 beneath the posterior region, where the process is inaugurated 

 (Text-fig. 3). 



The third point suggested was the effect on further develop- 

 ment of the early formation of an amnion from the unspecial- 

 ized posterior ectoderm. It is interesting to observe, as the 

 fourth diagram (as well as the final plate in this paper) shows, 

 that after the formation of the amnion as one of its organs, the 

 ectoderm, as well as the mesoderm beneath, continues to grow 

 posteriorly, carrying the amnion behind and budding anteriorly 

 the ectodermic portions of the segments of the body until, 

 finally, we reach a stage identical with that of the myriopod or 

 apterygote. The unspecialized tail-piece of this stage was 

 formed in the usual manner from the original, undifferentiated, 

 posterior tissue of the primary rudiment, from which the amnion 

 arose at an earlier stage. 



In a sense the formation of the ectoderm of the tail-piece, in 

 this later stage of the winged-insect embryo, may be thought of 

 as a regeneration of the lost terminal material which went into 

 the amnion ; just as a piece of the ectoderm of a developing 

 hydra (worm, or other form) might be removed at an early 

 stage, without disturbing the further development of parts from 

 the ectoderm, since the ectoderm remaining would supply the 

 loss. This statement must, however, be accepted as an illus- 

 tration of regeneration from undifferentiated tissue, only in so 

 far as such a process is comparable to normal growth following 

 the differentiation of an organ from unspecialized tissue. 



From this point of view the avinion is not a substitute for, or 

 a transformation of a posterior region of the myriopod body. It 

 is not derived from any previous structure. It is a specialized 

 structtire folded off in the winged-insect embryo, for some 

 adaptive reason similar to those causijig the dotib ling-tip of the 



