History of a transient nervous apparatus in certain Ichthyopsida. 395 



Developmental studies, on the nervous system chiefly, have now 

 engaged my attention for upwards of thirteen years, and among the 

 problems always kept in sight with a view to solution has been that 

 of the original mode of nerve-formation after ganglion-cells had been 

 evolved; in other words, that of the evolution of nerve. A return 

 will be made to this subject at the end of the present section. 



Speaking generally of published observations on the development 

 of nerve-fibres, one is struck by the circumstance that all those, who 

 have most ardently supported the "cell-chain" theory, have appealed 

 to lower forms of Vertebrates, or to the Invertebrata , in support of 

 their contentions. In this connection the researches of Balfour, 

 Van Wijhe, von Kupffek, Götte, Dohrn, Apathy and myself may 

 be cited. The most prominent advocates of the "process" theory have 

 mainly carried out their observations on Mammals, and here the 

 honoured names of Kölliker and His may be mentioned, as also 

 those of three younger observers, the lamented W. Vignal, M. von 

 Lenhossek, and ray friend Arthur Robinson. True, His has also 

 recorded observations on Elasmobranch embryos in support of the 

 "process" theory, and Dohrn has cast doubts on the correctness of 

 his own latest and important contribution to the question. 



Regarding Dohrn's actual position one is in some perplexity. In 

 one of the most brilliant of his studies he had built up an impregnable 

 stronghold on the side of those who maintain the "chain" theory, 

 only to desert his erection, soon after its completion, with the brief 

 intimation that he had discovered it to be built on a quicksand. 



After Dohrn's declaration of his renewed doubts, the publication 

 of a new study, in which all the facts of the last memoir should be 

 proved to be in conformity with the "process" theory, could only be 

 looked forward to with extreme interest. Anyone who had devoted 

 time and pains to the study of Elasmobranch embryology could not 

 but be very curious to see a work on this group, proving that the 

 nerves of these fishes always arise as processes of ganglion-cells. For, 

 surely, it is of some significance to note that, apart from Dohrn and 

 His, no student of Elasmobranch development has ever been able to 

 make out that the spinal and cranial nerves are there laid down in 

 any other form than as cell- chains! From neither of these eminent 

 embryologists has there yet appeared an extended work, with even 

 one plate of figures, in which the "chain" mode was disproved. 



In the development of the nerves of Raja^ apart from those of 

 the transient system, I also have seen things which seemed so far 



