TEOPHOBLAST AND SEROSA. 605 



Such are the considerations by which I seek to establish a 

 phyletic relationship between the trophoblast^ as it is presented 

 to us in the embryos of Peripatus novse-britannise, and the 

 serosa of insect embryos^ and for my part I am disposed to be 

 content with this demonstration. Nevertheless a few words 

 on the origin of the insect amnion may not be out of place. 

 In accounting for the serosa we have not, eo ipso, accounted 

 for the amnion, notwithstanding that they are both derivatives, 

 and inseparable derivatives, of the blastoderm. The amniotic 

 cavity requires special treatment. 



Until Heymous discovered the amnion and serosa of 

 Lepisma^ he supposed that the embryonic membranes were 

 to be regarded as a new acquisition of the Insecta Ptery- 

 gota, and that there was no basis upon which to frame any 

 hypothesis as to their phylogenetic history (2). 



For statements of their own views and references to the 

 views of others, the reader would find it well worth while to 

 consult the well-known memoirs of Wheeler and Heymons. 

 Suffice it to say here that Heymons (3) has conclusively shown 

 that the superficial or ectoblastic type of embryonic tract 

 as exemplified in Orthoptera is more primitive than the 

 immersed or entoblastic type as exemplified in Libellulidse 

 and Ephemeridae. 



So far as I have been able to form an opinion from the data 

 which are to hand, I think that the amniotic cavity of 

 insect embryos was originally a product of invagina- 

 tion, and that this invagination was primitively de- 

 rived from and associated with a ventral flexure of 

 the embryo. 



As already mentioned_, Heymons has in fact found that the 

 amniotic cavity of Lepisma is formed by invagination, which is 

 brought about by the ventral flexure of the embryo, the orifice 



strikingly calls to mind that of the Chilopoda, in tiiat a dorsal flexure of the 

 embryo precedes the ventral flexure. 



The Apterygota ectognatha, as exemplified in Lepisma, recall in their 

 embryonic development that of the Diplopoda in respect of the primary 

 ventral flexure of the embryo. 



