259 
same way as in Annelids prolongations of the trunk meso- 
blast secondarily penetrate into the prostomium (p. 212). 
Evidently no primary phylogenetic significance can be attri- 
buted to this mesoblast of the dorsal part of the snout of 
Amphioxus (the ventral half is filled up by the right 
anterior head cavity). And when RABL (1889) compares 
FRORIEP's unsegmented head-mesoderm of Craniates to 
this prolongation of the first somite in Amphioxus, this 
comparison can only lead to a negation of the fundamental 
significance attributed to this head mesoderm by FRORIEP. 
nion. — The conception, that the prostomium in 
Vertebrates does not contain any primary head mesoderm, 
seems to me to be supported also by the presence of the 
so-called “prcamnion” (VAN BENEDEN and JULIN, 18846) in 
Amniotes. The absence of mesoblast in the prostomium 
manifests itself also in the extra-embryonic area: the extra- 
embryonic mesoderm, being the continuation of the somatic 
or coelomesoblast, ends abruptly anteriorly at both sides 
of the embryo with a straight line perpendicular to the 
body axis and intersecting the latter at the limit of prosto- 
mium and soma. In front of this line no mesoderm is 
present and the amnion-folds consist only of ectoderm and 
entoderm (“pro-ammion”). Only afterwards is the anterior 
part of the extra-embryonic area, corresponding to the 
prostomium, invaded by the mesoblast. 
l am inclined to the view that the mesenchyme of Cteno- 
phores, Plathelminths and Nemerteans, of which the primary 
or larval mesoblast in Annelids and Molluscs represents 
a last vestige, has wholly disappeared in Vertebrates, where 
only the coelomesoblast, so strongly developed already 
in Annelids and perhaps to be traced back to the genital 
follicles of mesenchymatous ancestors, has remained. To 
the prostomium of Chordates, accordingly, no primary meso- 
derm belongs; the unsegmented head-mesoderm of FRORIEP 
is to be considered as the anterior part of the somatic or 
coelomesoblast in which segmentation has been suppressed, 
as we see it pass before our eyes in the branchial region 
of Elasmobranchs. The gill-slits pierce through that part 
of the lateral plate, that belongs to the head-region and 
from which, as VAN WYHE (1882) first emphasized, the 
Primordial gill-muscles are derived. The unsegmented mass 
of mesoderm, observed by DE LANGE in Megalobatrachus 
(“Urmesoderm”, 1913, p. 250) and which only secondarily 
