History of a transient nervous apparatus in certain Ichthyopsida. 321 



,^And so, from hour to hour, we ripe and ripe. 

 And then, from hour to hour, we rot and rot. 

 And thereby hangs a tale" — . Shakespeare. 



The object of the present contribution is to give a somewhat 

 detailed account of the developmental origin, growth, and fate — des 

 Werdens und Vergehens — of a system of ganglion-cells and simple 

 nerves of an evanescent character in the enibryological history of one 

 of the lower Vertebrates. 



The system arises at a very early stage of the development, 

 making its appearance in close association with what at a later stage 

 is destined to become the central nervous system of the adult Ich- 

 thyopsid, it persists for a time in the individual life-history, and, 

 finally, atrophies and dies away. 



As will appear in the sequel, this transitory apparatus possesses 

 well-marked morphological characters of its own — characters which 

 negative any suggestion that it might be, like the pronephros of certain 

 Vertebrate embryos, merely a precocious development of some struc- 

 tures appertaining to the permanent nervous system. 



At a subsequent stage the morphological nature of this evanescent 

 system would be discussed, and here, by anticipation, the conclusion 

 arrived at may be briefly indicated. 



In many animals where two distinct nervous systems of different 

 morphological characters appear in the life-history of the individual 

 the phenomena are in association with a metamorphosis, and here 

 also. An attempt to establish for the Vertebrata the principle of a 

 metamorphosis during the individual development would be something 

 in antagonism with all the tenets of modern embryology. But, if the 

 unadorned facts of development should point in such a direction, only 



