TROPHOBLAST AND SEROSA. 589 



Trophoblast and Serosa. A Contribution to the 

 Morphology of the Embryonic Membranes of 

 Insects. 



By 



Artliiu- llVilley, D.^cLond., Hon. M.A.Caiita1>., 



Balfour Student of the University of Cambridge. 



The substance of the following remarks formed the subject 

 of a paper ^ read by me at the last meeting of the British 

 Association in Bristol. 



It is matter of common knowledge that, during the early 

 stages of development, the embryos of insects are protected 

 by two membranes, — an inner, the amnion, and an outer, the 

 serosa, — which resemble in principle the homonymous foetal 

 membranes of the higher vertebrates. 



Both in the embryos of the Vertebrata Amniota and in those 

 of insects, the amnion and serosa are derivatives of the extra- 

 embryonic blastoderm (abstraction being made of the adven- 

 titious mesodermal elements which accompany the membranes 

 in vertebrates), and in both it may be stated in general terms 

 that the amnion is subsidiary to the serosa. In insects the 

 secondary character of the amnion, as compared with the 

 serosa, is particularly clearly marked, and it has been recog- 

 nised that the serosa, being a direct derivative of the blas- 

 toderm, is the older structure (Heymons, 4). 



The peripheral, extra-embryonic, non-formative epiblast or 

 blastoderm of the mammalian blastodermic vesicle has been 



' Entitled " Considerations bearing upon the Phylogeny of the Arthropod 

 Amnion." 



