^8 WHEELER. [Vol. VIII. 



should persist, and instead of sinking into the yolk, envelop 

 the whole &gg, secrete a granular and thereupon a chitinous 

 layer, and finally, during revolution take on the function of a 

 true serosa. That the organ is rudimental 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 somewhat complicated later development and by its sur- 

 vival in but very few forms out of the vast group of Ptery- 

 gotous insects. This seeming paradox may be explained, if 

 we suppose that the indusium was on the verge of disappear- 

 ing, being the last rudiment of some very ancient structure. 

 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. 



Pursuing the homologies of the indusium still further we 

 come to the Arachnida, where we find in the primitive cumulus 

 of spiders a structure comparable in many ways to the 

 Podurid "micropyle," as v. Kennel ('85, '88) and Lemoine ('87) 

 have suggested. There is, however, so much difference of 

 opinion regarding the position and signification of the primitive 

 cumulus that I should hardly be willing to agree with these 

 authors, were it not for two of Clapar^de's figures of the 

 PJiolcus embryo ('62, Figs. 6 and 7, PI. I). These show in the 

 median dorsal line a thickening which forcibly recalls the 

 "micropyle" of Anurida. Still it must be admitted that 

 Claparede has failed to prove the identity of this thickening 

 with the primitive cumulus. 



In Pentastomids the " facette " or "cervical cross" described 

 by Leuckart ('60) and Stiles ('9l) is very probably the homo- 

 logue of the crustacean dorsal organ and the insect indusium. 



Although no homologous structure has yet been detected in 

 the Myriopoda, the occurrence of a dorsal-organ-like structure 

 in such widely separated groups as the Hexapoda, Araneina, 

 Pentastomidae, and Crustacea is sufficient reason for regarding 



