History of a transient nervous apparatus in certain Ichthyopsida. 403 



Its degeneration at a period coincident with the commencing 

 histological differentiation of the permanent nervous apparatus is also 

 very significant. There remains to be considered the possibility that 

 the transient apparatus, like the pronephros, might be merely an 

 earlier developed portion of the permanent apparatus. In a foot-note 

 on page 399 several, to my mind, weighty objections have been urged 

 against this view. 



To them might be added the facts of the gradual suppression of 

 the transient system with the initiation of uterine development, even 

 among fishes, and its apparent absence in all forms above the Ichthyopsida. 

 On, what I may perhaps term, von Kupffer's view of its nature these 

 facts appear to admit of no reasonable explanation. 



Founding upon the detailed account of its morphological nature 

 given in the preceding pages, it may be briefly denied that there exists 

 any morphological resemblance whatever between the two systems. 

 The one precedes the other in the development, and is as unlike its 

 successor in all morphological respects as the nervous system of an 

 Invertebrate larva is different from that of the mature organism. 



Every morphologist has a very definite conception of the points 

 which characterise the Vertebrate nervous system, but can any mor- 

 phologist state that in the transient nervous apparatus of Baja and 

 other forms he encounters characters, which justify a conclusion that 

 the system is that of a Vertebrate animal? 



Do not both developmental and comparitive histories demonstrate 

 it rather to be larval in nature? 



Preliminary remarks on Development Iby suI)stitution of 

 organisms, or antithetic alternation of generations. ^) 



It is one of Kleinenberg's services to have established the 

 principle of the development by substitution of organs — "die Ent- 

 wicklung durch Substitution von Organen". This, even 

 more than the principle of change of function, whose discovery we 

 owe to DoHRN, forms a foundation-stone of the Science of Com- 

 parative Embryology. 



But, possibly, there may be another and parallel process of far- 

 reaching importance, and this might be termed "development by 

 substitution of organisms". In other words, as elsewhere in- 



1) For more recent developments of the questions here discussed 

 see three short papers by Mukbay and myself, in : Anat. Anz., Bd. XI, 

 1895, p. 234—255. 



Zool. Jahrb. IX. Abth. f. Morph. 27 



