NO TES. 



103 



to be present in the spinal cord of all those Vertebrates whose tail 

 serves as an important organ of locomotion. Thus, they occur in 

 fishes, tailed Amphibia, in the tadpoles of tailless Amphibia, and, 

 finally, they have been recently discovered by Max Koppen in the 

 caudal region of the spinal cord of the lizard. In the frog and higher 

 forms they do not occur. From these considerations Koppen 

 tliinks that there is a causal relationship between the occurrence 

 of giant-fibres in the spinal cord and the presence of a locomotor 

 tail. The caudal locomotion, characterised by the rapid swaying 

 motion of the tail, is not confined to the post-anal region in 

 Amphioxus, but involves the whole body. 



Contrary to the observations of Eisig, both Nansen and Rohde 

 are of opinion that the giant-fibres of Annelids (Polychasta) have 

 the same physiological significance for the central nervous system 

 as those of Amphioxus. 



Some of the older authors mistook the giant nerve-fibres for 

 capillary blood-vessels. As a matter of fact no blood-vessels 

 traverse the central nervous system of Amphioxus. It may be 

 added, also, that there are no meduUated nerve-fibres. 



II. (p. 95.) Several suggestions have been made as to pos- 

 sible representatives of the spinal ganglia of the dorsal roots of 

 the Craniota in Amphioxus. 



Omitting earlier, and obviously erroneous, suggestions, Rohde 

 (1888) regarded the nuclei, which he found imbedded in the 

 dorsal roots, as a collection of " nervous nuclei," comparable to 

 the spinal gangha of the higher Vertebrates (Fig. 46). According 

 to Retzius (1890) these nuclei are not of a nervous nature (prob- 

 ably belong to supporting-cells), and he tentatively suggests that 

 the spinal ganglia are represented by groups of bipolar ganglion- 

 cells which occur inside the spinal cord at fairly regular intervals 

 in two longitudinal rows, one on each side of middle line. The 

 main process (axis-cylinder) of these cells divides in T-form, and 

 one of the branches of the T passes into the dorsal root. (Cf. 



Fig- 50.) 



Finally, Hatschek (1892) finds the homologues of the spmal 

 ganglia at the points where the dorsal nerves divide into 7-avuis 

 dorsalis and j-amus ventralis. 



