70 THE ANCESTRY OF VERTEBRATES 



coalesced to form the head. Hatschek (1909, p. 498) 

 assumes first the formation of a dorsal part of his acro- 

 merite by a forward outgrowing and overturning of- the 

 fore-end of the brain, as described previously (cf. p. 50), 

 then the formation of a ventral part by the growing out of the 

 maxillary-palatine processus of the first visceral arch. The 

 adherents to the second view, on the contrary, consider 

 the head region as a part of the body, which from the 

 beginning must be opposed to the segmented trunk. 

 De LanGE (1913) e. g. joins HUBRECHT (1902, p. 70) in the 

 view, that the trunk grows out from the head in the same 

 way as the segmented soma of Annelids grows out from 

 the trochophora-larva. Indeed among students of Annelid 

 development we meet the same controversy: most of them, 

 with Kleinenberg (1886), oppose the prostomium to the 

 segmented trunk, which has grown out secondarily; on the 

 other side, however, we may point to the fact, that in 

 asexual generation the prostomium is formed as an out- 

 growth from the first segment. 



Now from the point of view of my theory FRORlEP is 

 quite right in emphasizing, that not the whole head of 

 Vertebrates has originated by the fusion of segments, but 

 that there has been an anterior unsegmented nucleus to 

 which afterwards somatic segments have been added. This 

 unsegmented anterior part corresponds to the prostomium 

 of Annelids, as was already anticipated by BALFOUR 

 who in 1881 wrote: "These considerations indicate with 

 fair probability that the part of the head containing the 

 fore-brain is the equivalent of the praeoral lobe of many 

 Invertebrate forms.'* On the -other hand, as regards the 

 backward extension of the prostomium, it seems to me to 

 follow from my theory, that the view of Gegenbaur, Van 

 Wyhe etc. is nearest the truth, and that the primarily unseg- 

 mented part of the head, already so much reduced in size 

 by FRORlEP's own researches on Elasmobranchs, does not 

 reach any further indeed than the praechordal part, as 

 suggested by BALFOUR. We are led to this decision especially 

 by the answer, which my theory gives to the second of the 

 above questions. 



Is there an unsegmented head-mesoblast? — Before again 

 considering this second question we have to deal with the 

 second part of the first: i.e. whether there is a primarily 

 unsegmented head mesoderm belonging to the prostomium. 



