ORIGIN AND STRUCTURE OF THE HEAD 73 



same way as in Annelids prolongations of the trunk meso- 

 blast secondarily penetrate into the prostomium (p. 26). 

 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 

 FRORlEP'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. 



Proamnion. — 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 "proamnion" (VAN Beneden andJULIN, 18846) in 

 Amniotes. The absence of mesoblast in the prostomium 

 manifests itself also in the extra-embryonic area: the extra- 

 embryonic mesode'rm, 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-amnion"). Only afterwards is the anterior 

 part of the extra-embryonic area, corresponding to the 

 prostomium, invaded by the mesoblast. 



I 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 



