546 KNOWER. [Vol. XVI. 



is the reason Wagner finds gland cells, muscle cells, etc., not 

 wandering out of the otherwise uniform layer. When a migra- 

 tion of specialized cells does take place, we shall have to look 

 further than to such a simple statement of inorganic physics 

 for the explanation. 



C. I shall not examine Willey's (29) hypothesis at length, 

 since, in as far as it refers to the origin of the amnion, it 

 appears to be largely a statement of Heymons's (15) views, 

 which will be considered further over. 



Willey's main thesis seeks to prove, by reversing a theory of 

 Hubrechts's, that the extra-embryonal blastoderm of the insect 

 Qgg {i.e., the serosa and amnion) is a secondary cellular mem- 

 brane, derived in a curious roundabout manner from a more 

 primitive, extra-embryonal trophic membrane, " the tropho- 

 blast"; which, "as it is preserved to us in the embryo of 

 Peripatiis novae-britaimiae, arose in adaptation to a viviparous 

 habit acquired by the terrestrial descendant of an aquatic 

 ancestor ; and that it became transformed, whether directly or 

 by substitution, into the serosa, in correlation with the second- 

 ary deposition of yolk-laden eggs." 



The following fundamental assumptions seem to me inad- 

 missible : That the viviparity of Peripatus is primitive ; that 

 " lecithality and deposition of the eggs of insects are both 

 secondary"; that this application of the idea of substitution in- 

 volving the reverse of Hubrechts's idea is reasonable; that the 

 amnion is a derivative of the extra-embryonal blastoderm in 

 insects ; or that the serosa of the insect ^g^ has any such indi- 

 rect phylogenetic history, believing it as I do to be directly 

 comparable to the inactive extra-embryonal surface cells (Deck- 

 schicht) of other yolk-laden eggs. 



D. \. I have already discussed, in the division of this paper 

 headed General Conclusions as to this Type of Insect Develop- 

 ment (page 29), certain aspects of the theory presented in 

 Korschelt and Heider's text-book. Reference must be again 

 made to that section of my paper, where the original sources 

 and criticisms of the theory are quoted. This theory, which 

 originated with Will and Wheeler (25), was later modified by 

 Heider. 



