No. I.] DESMOGNATHUS FUSCA. 263 



representing a section through the Gasserian ganglion of a 

 larval lamprey, has been introduced in order to show the 

 various transitional stages. 



In the cells of the neuraxis of the DesmognatJms, there is 

 apparently an exact reversal of transition. Starting with the 

 layer next adjacent to the endymal cells, we find the unipolar 

 condition. Toward the ectal boundary of the cinerea, the cell 

 process becomes very much shorter, until at the very margin 

 of the cinerea the bifurcation of this process occurs at the 

 cell itself, Figs. 44 and 49. The cells lose their pear-shaped 

 appearance as if from ento-ectal pressure and assume a fusi- 

 form outline with a branching process extending from each 

 end. Figs. 56, 58, 60. In other instances, there has been the 

 appearance of a pyriform cell with one of its broad sides 

 facing the alba, from which one or more processes grow out. 

 Figs. 56, 58. 



The conditions are so suggestive of the embryology of the 

 neuraxis of higher animals as described by His, that a brief 

 recapitulation may be permitted. The germ cells which give 

 rise to the neuroblasts lie near the cavity between the spongio- 

 blasts or supporting structures. The protoplasm of the neuro- 

 blasts arranges itself on the side of the nucleus, away from the 

 cavity, and this extending toward the surface becomes in the 

 course of growth, the nerve fiber. The neuroblasts themselves 

 migrate toward the periphery to form the mantle layer or 

 cinerea ; the fibers lying between the cinerea and the ectal 

 surface represent the alba. 



An interesting point arises in connection with the cells 

 lying intermediate between the endyma and alba. According 

 to Roller's idea (44) they might be regarded as nerve cells 

 developing from endymal cells. Embryological evidence, as 

 shown by His, seems to refute this method of evolution. The 

 location of these intermediate cells would indicate that they 

 were true nerve cells undergoing a change of form, in case of 

 their migration toward the periphery ; or, if the migration has 

 occurred before the cell has developed its process, its form 

 may be influenced by the direction of the growth of the pro- 

 cesses, — these mav follow the lines of least resistance. The 



