212 Mr. F. Nansen on the Histological Structure 



tigated ; it was only within the last few years that some light 

 was thrown upon it especially by the writings of Prof. E. 

 van Beneden and Dr. C. Julin *, but it still remains very 

 backward. The central nervous system consists of the hraitij 

 situated between the apertures of the mouth and the cloaca, 

 from the posterior end of which there issues the dorsal gan- 

 glionic cord (" cordon ganglionnaire visceral ou dorsal ") 

 described by Van Beneden and Julin, which extends back- 

 wards past the cloacal aperture until it disappears in the 

 neighbourhood of the liver f. 



Hitherto it has been the brain that has particularly inter- 

 ested me. With it I have tried many different modes of 

 treatment, but none has as yet given satisfactory results ; of 

 course, I have employed all the ordinary methods with osmic 

 acid, chromic acid, bichromate of potash, bichloride of mercury, 

 &c., as also staining with carmine, hgematoxyline, aniline 

 colours, &c., and I have further made trial of Golgi's method 

 (with bichromate of potash and nitrate of silver) , but without 

 obtaining the desired results, so that it will now be my en- 

 deavour to find out new methods. Golgi, in his method, has 

 found one which, as regards the Mammalia and the higher 

 Vertebrata, furnishes images of such striking distinctness that 

 we cannot wish for better ; the point now is to discover a 

 method which may give similar results in the case of the lower 

 animals, and only then can we give any more satisfactory 

 answers to the questions above formulated. 



* C. Julin, '' Eecli. s. I'Organisation des Ascidies simples," in Arch, 

 de Biol, tome ii. (1881), pp. 51)-126; E. van Beueden et 0. Julin, "Le 

 systems uerveux central des Ascidies adultes," in Arch, de Biol, tome v. 

 (1884). 



t My investigations of this cord have as yet been quite superficial ; 

 but it Avould appear as if it may have a somewhat different structure in 

 the different species. In the species which I have especially examined 

 i^Phalkisia venosa, P. mentula, P. ohliqua, Ascidiu scahra, Corclla parallelu- 

 (jramma) it has only a small development : the ganglion-ceUs generally 

 are slender, elongated (bipolar), and closely packed together; it is but 

 seldom that they are of the size of those found by Van Beneden and 

 Julin in the species of Mohjnla (M. anipulloides) investigated by them ; 

 further, it appears that their arrangement and position may differ con- 

 siderably from what the above-mentioned naturalists found in Molgtila. 

 It was but rarely that one could perceive any tendency in the cells to an 

 arrangement about a common central axis, towards which their processes 

 were directed ; on the other hand, in several species, at any rate, I 

 have found two fibrillar cords running one on each side of the principal 

 mass of the cells, by which means the cells therefore come to occupy the 

 middle of the ganglionic cord, a position which, regarded superficially, 

 mif^ht remind one of the position of the cells in the spinal cord of the 

 Vertebrata. I believe that I have several times seen nerves going off 

 from the ganglionic cord. 



