256 
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. 236), 
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 FRORIEP 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 FRORIEP’'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 
y 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. 
