257 
The cell-lineage investigations have taught us that, though 
KLEINENBERG'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 entódermal origin. The latter has a bilateral 
character and gives rise in Arnelids to the double series 
of coelomic segments. The former has a more or less 
radial *) origin and a meser.chymal 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 ccelomic pouches from the genital follicles 
of the latter (cf. p. 211). Thus the ectomescblast, 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 ccelomesoderm 
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 
o 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. 212). 
an such an unsegmented mesoblast be recognized also 
in Vertebrates, is this the “head-mesoderm” of FROR:EP ? 
HJBRECHT (1890, 1908), MARCUS (1910, p. 111) 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), 
