History of a transieut nervous apparatus in certain Ichthyopsida. 397 



The term will possibly meet with less opposition when it is reco- 

 gnised that the existence of the process is warranted by ascertained 

 facts, and that one may believe it to take place without prejudice to 

 any general ideas of nerve-formation. 



It is likely that search in the literature of embryology would 

 bring to light many examples of this process, here one only need be 

 cited 1). In their monograph on "Die Actinien" (p. 76) 0. and 

 R. Hektwig say : "einige (Ganglienzellen) sind so klein, dass ihr Körper 

 von fast nichts anderem als dem Kern gebildet wird etc." And a 

 reference to tab. 6 of their memoir shows that many of the cells 

 figured are strikingly like the small series about to be described here. 



Among the ganglion-cells of the apparatus there are many bipolar 

 ones in the mesoderm whose nerve-processes, as already described, 

 pass in two directions. Examples have been figured in Fig. 15, 

 plate 22, Figs. 31, 35, plate 23, and Figs. 43, 46, plate 24. 



What may be regarded as the most primitive of these is the 

 case of a bipolar-cell, like that in Fig. 43 or Fig. 46, in which, each 

 of the two nerve-processes ceasing in the ganglion-cell, there is no 

 direct passage through the cell. Such a cell must receive the impulse, 

 and then transmit it along the other fibre. In other words the cell 

 must be a centre for reflex-action. In a primitive nervous apparatus, 

 just beginning to be centralised, if the nerve-processes of ganglion- 

 cells are in contact, the impulse would be transmitted from cell to 

 cell without passing directly through any of them. 



A step in advance is where a direct passage is formed through 

 the cell by making up the deficiency of nerve-fibre between the ter 

 minai ends of the two processes. The nerve-fibre is then continued 

 right through the cell, as shown in Figs. 15 and 31 ; and the ganglion- 

 cell, although the parent of the nerve-fibre, becomes almost an ap- 

 pendage of it. The matter reaches its climax, where, as in Fig. 44, 

 the original ganglion-cell has become reduced, in consequence of loss 

 of specific functions, to a mere nucleus embedded in the fibre to 

 which it has given origin. 



Attention may now, be called to Figs. 79, 80 and 81 of plate 26. 

 Considerations of space in the plates led to a resolution to refrain 

 from publishing figures of the sections from which the cells shown in 



1) Of course, what is known of the great variations of form in the 

 ganglion- cells of the brain and sense-organs of higher forms is not 

 forgotten. 



