TROPHOBLAST AND SEROSA. 601 



he found that each possessed two separate iridusial thickenings 

 of the serosa or blastoderm. 



In its first appearance as a local thickening of the serosa, 

 the indusium has essentially the same topographical relations 

 with regard to the nuchal region as the dorsal organ of the 

 Poduridse. 



Wheeler says (12, p. 56) : '' Although much simpler in its 

 structure, I do not hesitate to homologise this ' micropylar ' 

 organ in Anurida and the Poduridte in general with the 

 indusium of Xiphidium.^' 



Again, with reference to the indusium Wheeler says (p. 58) : 

 " That the organ is rudimeutal is shown by its tendency to 

 vary, especially during the earlier stages of its development; 

 that it still performs some function is indicated by its some- 

 what complicated later development, and by its survival in but 

 very few forms [Locustidse] out of the vast group of Pterygo- 

 tous insects. This seeming paradox may be explained if we 

 suppose that the indusium was on the verge of disappearing, 

 being the last rudiment of some very ancient struc- 

 ture. As such a rudiment it no longer fell under the influence 

 of natural selection, and for this reason began to vary consider- 

 ably like other rudimental organs. Some of these fortuitous 

 variations may have come to be advantageous to the embryo, 

 and were perhaps again seized upon by natural selection, the 

 nearly extinct organ being thus resuscitated and again forced 

 to take an active part in the processes of development." 



Eventually the indusium undergoes retrogressive develop- 

 ment like the serosa of other forms, and produces an indusial 

 dorsal organ, which, however, is not absorbed but shed 

 (Wheeler [12]). 



Accepting Wheeler's view of the homology between the 

 dorsal organ of Poduridaj and the indusium of Locustidse, and 

 admitting that these structures are not the same as the serosal 

 dorsal organ of other insects, we are now in a position to 

 discuss the bearing of these various observations. A certain 

 amount of repetition is unavoidable if there is to be any 



