No. 3-] STUDIES ON THE HETERONEMERTINI. 431 



the structure of the nervous elements in general ; any one 

 wishing to consult the rather voluminous literature on this sub- 

 ject may find more or less complete bibliographies given by 

 H. Schultze ('79), Eisig ('87), Nansen ('87), Rohde ('90), and 

 Friedlander ('89). But one point may be mentioned, namely, 

 that the newer researches are tending to prove that the nervous 

 element of the so-called " dotted substance " is a nerve tubule 

 dead, not a. nerve fibril ; and that the fibrillar elements of this 

 substance are derivatives of the neuroglia, or of other connec- 

 tive tissues. Whether, however, as Leydig ('85) maintains, the 

 hyaloplasm alone represents " die eigentliche Nervenmaterie," 

 we are not as yet in position to decide ; for the nerve tubule 

 consists not only of hyaloplasm (though this substance may 

 preponderate), but also of spongioplasm. And unless one would 

 side with Rohde ('90a, 92) in assuming — what seems very im- 

 probable — that the spongioplasm of the ganglion cell is formed 

 entirely by neuroglia fibres, we must consider the spongioplasm 

 as much a vital substance of the cell as the hyaloplasm, and 

 therefore as much a vital part of the nerve tubule. Further, 

 the dotted substance would appear not to be a " spongy mesh- 

 work " with confluent meshes, but to consist of nerve tubules 

 (which do not anastomose), between which a fibrous neuroglia 

 network is situated. 



The structure of the axis cylinder has also been a subject of 

 much controversy, and has been described (i) as homogeneous, 

 (2) as fibrillar, (3) as consisting of a number of parallel " primi- 

 tive nerve tubes," (4) as having a honeycombed meshwork 

 structure. This difference of opinion is due to two reasons : 

 (i) the study of different objects, and (2) the use of different 

 technical methods on the part of the observers ; for seldom has 

 an investigator studied a number of different organisms and 

 at the same time employed various reagents. Now because I 

 have found the axis cylinder of the nemerteans to be homoge- 

 neous, it would be false reasoning for me to conclude that this 

 structure holds good also in other forms. On the contrary, 

 the researches on the vertebrate axis cylinder would show it to 

 be fibrillar (with "primitive fibrils" enclosed in hyaloplasm); 

 while in some of the invertebrates it would seem to be fibrillar, 



