66 THE ANCESTRY OF VERTEBRATES 



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 ZlEGLER (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 Korpers 

 bilden sich Kiemenspalten und Kopfsinnesorgane, als Folge 

 hiervon vergrossert sich das Centralnervensystem zum Gehirn. 

 Bei der machtigen Entfaltung aieses Gebietes kommt es dann 

 zur Zerstorung von Somiten des vorderen Korperendes 

 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 iibrige Korper. Von der 

 alten Wirbeltheorie und ihrer modernen Nachfolgerin, der 

 Segmenttheorie des Schadels und Kopfes, ist nichts mehr 

 iibrig geblieben." 



FRORIEP versus GEGENBAUR and VAN WYHE.— Which 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 



