History of a transient nervous apparatus in certain Ichthyopsida. 401 



few deeper lying ones of other forms would demand explanation. 

 There are also in other fishes what are known as "Mauthner's fibres", 

 and sometimes large giant ganglion-cells in connection therewith, of 

 which an admirable account is given in von Kölliker's "Gewebe- 

 lehre". All these elements may possibly be referable to the same 

 category, but a detailed consideration may be postponed for another 

 occasion, when what is known of similar structures in Invertebrata ^) 

 would also fittingly come under review. 



Degenerate condition of the apparatus in Raja. 



In the preceding pages the fact has more than once been inci- 

 dentally commented upon that the apparatus is subject to individual 

 variations. These are of two kinds ; the one concerns the general 

 state of its development as a whole, the other its parts. 



Some embryos exhibit a greater richness of ganglion-cells and 

 nerves than others do, and this would be especially accentuated, if 

 some of the embryos described were to be closely compared with others 

 which were rejected because of a lower grade of development presented 

 by the transient apparatus. 



Although one meets again and again with similar pictures in the 

 nerves of the system, no two embryos are exactly alike in the detailed 

 characters of the apparatus. It cannot be said that any of these 

 variations affect to an appreciable extent that part of it which lies at 

 the summit of the cord. It is the nerves of the system, the vagrant 

 ganglion-cells in the mesoderm , and those in the myotomes which 

 vary most. P\irthermore, it has been put on record that here and 

 there individual cells, or groups of such, begin to degenerate in very 

 early stages long before general atrophy has set in. Odd ganglion- 

 cells are encountered, even when the apparatus is at its prime, which, 

 having apparently no connection with other parts of the system, seem 

 to be without any justification for existence. 



There is, moreover, no very apparent symmetry of the apparatus ; 

 the ganglionic groups in the mesoderm seldom correspond on the two 

 sides, and the nerves, though often obviously paired, are not in- 

 variably so. Even if paired , the one nerve may have a ganglionic 

 composition very different from that of the other, on which there may 

 be fewer or no ganglion-cells. It rarely happens that, where there is a 



1) Of our knowledge of these excellent summaries have been given 

 by Spengel and Eisio. 



