VIII.] HOMOLOGIES. 191 



But even if this were as asserted, it nevertheless fails to 

 explain the peculiar condition presented by Syllis and 

 some other annelids, where a new head is formed at 

 intervals in certain segments of the body. Here we have 

 evidently an innate tendency to the development at 

 intervals of a complex whole. It is not the budding out 

 from, or spontaneous fission of, certain segments ; but the 

 transformation in a definite and very peculiar manner of 

 parts which already exist into other and more complex 

 partSj. Again, the processes of development presented by 

 some of these creatures do not by any means point to an 

 origin through the linear coalescence of primitively dis- 

 tinct animals by means of imperfect segmentation. Here 

 reference may be made to those flies before spoken of (see 

 ante, p. 51), in the development of which the legs, wings, 

 eyes, &c., are derived from masses of formative tissue 

 (termed imaginal disks), which, by their mutual approxi- 

 mation, together build up parts of the head and body, 1 

 recalling to mind the development of Echinoderms. 



Again, Nicholas Wagner found in certain other Diptera, 

 the Hessian flies, that the larva gives rise to secondary 

 larvse within it, which develop, and burst the body of the 

 primary larva. The secondary larvse give rise, similarly, 

 to another set within them, and these again to another 

 set. 2 



Again, the fact that in Tcenia echinococcus one egg pro- 

 duces numerous individuals, tends to invalidate the argu- 

 ment that the increase of segments during development 

 is a relic of specific genesis. 



Mr. H. Spencer seems to deny serial homology to the 



1 Professor Huxley's Hunterian Lecture, March 16, 1868. 

 3 Ibid., March. 18. 



