of the Central Nervous System in Ascidia &c. 211 



cesses, after giving off small side-branches, pass directly into 

 a peripheral nerve and form a peripheral cylinder ; Z>, second 

 type, in which the nervous processes divide up into small 

 branches, which lose themselves in and contribute to form the 

 fibrillar web ? 



3. Do the nerve-cylinders in the peripheral nerves consist 

 of two kinds : — a, in the first place, of nerve-cylinders, which 

 originate directly from nerve-cells and constitute continuations 

 of the nervous processes of the latter ; b, in the second place, 

 of nerve-cylinders, which originate from the fibrillar net, and 

 are produced by a union of many small branches ? 



4. Do certain nerve-cells send out their nervous processes 

 directly into the peripheral nerves without their passing 

 through the fibrillar mass of the central nervous system, and 

 can nerve-cells occur in the course of the voluntary nerves ? 



5. What is the case of the primitive fibrillse described by 

 Hermann *, Hans Schultze f, and others ? are these a really 

 existent nerve-element, and how are they to be regarded ? 



6. In what relation do the nerve-elements of the Invertebrate 

 animal stand to those of the Vertebrate? are the nerve-tubes 

 ("tubes nerveux "), or, as I have called them, "nerve- 

 cylinders," of the Invertebrate homologous with the " axis- 

 cylinders " of the Vertebrate animal ? 



These are questions of no small importance which still wait 

 for a satisfactory scientific solution. They are, however, 

 questions the answers to which, at all events in part, still lie 

 upon the limits of the range of modern microscopy, and, espe- 

 cially in what relates to the nervous system of Invertebrata, 

 we have such great technical difficulties to contend with that 

 we can only hope that for some time to come we shall probably 

 have to content ourselves more or less with assumptions. The 

 point is to find new and certain methods of investigation quite 

 different from those which have hitherto been generally adopted. 



My investigations to the present time can therefore make 

 no claim to give a satisfactory solution of these questions ; 

 they must be regarded only as a tentative effort to climb over 

 a hill, which certainly is not insurmountable, but which it 

 will need severe labour to get right over. 



Central Nervous System of the Ascidia. 



The histological structure of the nervous system of the 

 Ascidia has remained until quite recently as good as uninves- 



* ' Das Centralnervensystem von Hirudo medicinalis : ' Munich, 1875 

 (Prize essay), 



t " Die fibrilliire Structur der Nervenelemeute bei Wirbellosen," in 

 .\i'ch. f. mikr. Auat. Bd. xvi. 1879. 



