ORIGIN AND STRUCTURE OF THE HEAD 71 



The cell-lineage investigations have taught us that, though 

 KLElNENBERG's (1886 p. 3): "Es gibt gar kein mittleres 

 Keimblatt" went a little too far, yet we cannot speak of a 

 middle germinal layer homologous throughout the whole group 

 of three layered animals. In the Zygoneura, mesoderm from two 

 different sources may be distinguished, the one of ectodermal 

 and the other of entodermal origin. The latter has a bilateral 

 character and gives rise in Annelids to the double series 

 of coelomic segments. The former has a more or less 

 radial ^) origin and a mesenchymal character. It is also known 

 as the larval mesoblast and was compared by MEYER (1890), 

 as we have seen, to the mesenchyme of Turbellarians, while 

 he derived the coelomic pouches from the genital follicles 

 of the latter (cf. p. 25). Thus the ectomesoblast, representing 

 in the larvae of such Coelomata as Annelids and Molluscs the 

 mesenchyme of mesenchymal worms and Ctenophores. in 

 the same way as the so-called head-kidney of these larvae 

 may be compared to the protonephridia of the flatworms, 

 is phylogenetically older than the ento- or coelomesoderm 

 and therefore has been termed by CONKLIN (1897, p. 151) 

 primary or radial mesoblast, in distinction to the latter 

 which he defined as secondary or bilateral mesoblast. While 

 the secondary mesoblast is restricted to the segmented 

 soma, the primary mesoblast does not belong exclusively 

 to the prostomium. It even originates from the ectoderm 

 of the hyposphere (3rd quartett of micromeres), from which 

 the soma is formed. It represents the mesoblast of the whole 

 larva and of the primary body cavity, to which belongs part 

 of the cavity of the prostomium in the adult form (cf. p. 26). 

 Can such an unsegmented mesoblast be recognized also 

 in Vertebrates, is this the "head-mesoderm" of FRORIEP? 

 HUBRECHT (1890, 1908), MARCUS (1910, p. Ill) and De 

 Lange (1913) think they can demonstrate for the latter an 

 origin different from that of the coelomic mesoderm, the 

 former being split off from' the entoderm, the latter being 

 of "animal" or ectodermal origin. This assumption, however, 

 is wholly based on the supposition that LWOFF (1894) 

 was right in considering the roof of the archenteron of 

 the Vertebrate gastrula as being composed of invaginated 

 ectoderm cells, a view which has found far from general 



') In Annelids this has been only definitely observed in Scoloplos 

 (Delsman, 1916). 



