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entoderm of the most anterior part of the archenteron-roof, 
the latter from the ectoderm which, according to LWOFF's 
(1894) conceptions, is invaginated round the dorsal blasto- 
pore border to form the whole, or, after MARCUS, nearly 
the whole, roof of the archenteron (cf. last chapter). The head- 
mesoderm (“Urmesoderm”) is compared by DE LANGE to the 
mesoderm of the radiate ancestors of the Bilateria, that is 
to the mesenchyme of the Ctenophores and flatworms, and 
of the trochophora. Truly, in doing so, he leaves out of 
consideration, that the cell-lineage investigations have led 
to the directly opposite view, that the mesenchyme of the 
radiate Invertebrates and of the larvae of bilateral forms 
like Annelids and Molluscs is to be considered as ecto- 
mesoderm, the segmented trunk mesoderm of Annelids on 
the contrary as entomesoderm This, however, is to be 
explained by the fact that DE LANGE probably has in his mind 
a derivation of Chordates from Deuterostomians. The con- 
troversy has continued until the present time between the 
views of GEGENBAUR, VAN WYHE etc. and those of FRORIEP. 
In still recent years both have found a champion, respectively 
in ZIEGLER (1915) and VEIT (1916), of whom the latter sum- 
marizes FRORIEP's views as follows: “In dem anfangs sehr 
kleinen ungegliederten Bezirk am Vorderende des Körpers 
bilden sich Kiemenspalten und Kopfsinnesorgane, als Folge 
hiervon vergrössert sich das Centralnervensystem zum Gehirn. 
Bei der mächtigen Entfaltung aieses Gebietes kommt es dann 
zur Zerstörung von Somiten des vorderen Körperendes 
mitsamt ihrer Nerven; es vereinigen sich die stark entwickelten 
Kopforgane mit der Chorda dorsalis und den Resten der 
Somite zum Urkopf, dem sog. Palaeocranium. Nach diesen 
Anschauungen ist der Kopf von der ersten Anlage an nicht 
in einer Weise gegliedert wie der übrige Körper. Von der 
alten Wirbeltheorie und ihrer modernen Nachfolgerin, der 
Segmenttheorie des Schädels und Kopfes, ist nichts mehr 
übrig geblieben.” 
Froriep versus Gegenbaur and Van Wyhe. — Wûich of 
these two opinions is now supported by my theory, which 
is best brought into line with it? Before considering this 
question some observations must still be made, which may 
contribute to reduce the gap separating both parties. On the 
one side GEGENBAUR (1887, p. 77) already granted, and 
VAN WYHE (1889) emphasized, that it is improbable that 
at least the palaeocranium has originated by the fusion 
