_J550 _ 



SOME and Thompson' evolve the lateral nerve of Selachians from "the 

 extensive and irregular commissural system connecting the posterior 

 roots of Amphioxus" the comparison is only on a level with the 

 older idea of Hasse who, ascribed to Amphioxus a highly organised 

 optic organ and nerve directly comparable to those of higher Verte- 

 brates. 



Another argument of van Wijhe's depends on the value which 

 can be attached to the division of embryonic life into somewhat 

 artificial periods. Any organ which does not arise before his "Acra- 

 uier-Stadium " must have arisen in the "Stamm" of the Vertebrates 

 themselves. 



This Acranier-Stadium of van Wijiie consists in the absence of 

 a "Trennung von Kopf und Rumpf ". It corresponds to stage H and I 

 of Balfoue. If this stage be really one of the ancestral stages of 

 Vertebrates, then such ancestors had a very bad time of it. There 

 are then present: — two to three gill clefts, the rudiments of eye 

 and ear, the mesoderm somites of head and body, a notochord, brain, 

 rudiments of nerves, and practically that is all. Every thing else, 

 including the limbs, must then have arisen in the Vertebrates themselves. 



The argument, like the one on the presence or absence of organs 

 in Amphioxus, is not valid, for again, if rigidly carried out, the con- 

 clusions it would lead to would be intolerable. 



I cannot say that van Wijhe is more fortunate in his idea of 

 phylogeny of the segmental duct. He says: "Die ersten Cranioten 

 besaßen keinen Vornierengang: die Vorniere mündete durch einen 

 Porus lateral von der Drüse nach außen aus. Diese Ötinung rückte 

 später nach hinten , und aus ihrem Außenrande entwickelte sich der 

 Gang, der, die Cloake erreichend, in dieselbe einmündete". 



It seems as though van Wijiie regarded the pronephros as some- 

 thing in its nature quite different from the rest of the segmental 

 nephridia (Urniere, Mesonephros) which, develop shortly after it, and 

 come to open into the pronephric duct. 



His conception leads to the conclusion that, in the ancestors of 

 Vertebrates, there was first one nephridinm on each side (pronephros), 

 that this, for some incomprehensible reason, developed a pronephric 

 duct for itself alone, that the pronephros degenerated, and in its stead 

 their were formed in segments immediately following it a number of 

 nephridia (Urniere, Mesonephros) which in their turn opened into the 

 pronephric duct. The whole process in inconceivable and very un- 



1) Zool. Anzeiger. No. 227. 1886. 



