116 



Dr. J. Beard. 



[May 16, 



the whole length of tlie spinal cord of Lepidosteus and certain other 

 fish, embryos. So little had hitherto been published about their 

 occurrence in different groups of fishes, that a comparative investiga- 

 tion of the matter seemed desirable ; all the more, as I soon arrived at 

 conclusions as to their meaning and fate very different from those of 

 Dr. Paul Mayer, the only author who has paid much attention to such 

 cells (in the Scylliidge). In all cases these giant ganglion cells 

 occupy the same typical position in the extreme dorsal or neural 

 border of the spinal cord. They are found in very young embryos 

 in nearly every transverse section through the region of the spinal 

 cord. Very often a pair, one on each side of the middle line, is met 

 with in a single section. To obtain a clear insight into their distribu- 

 tion horizontal longitudinal sections are necessary. When the sections 

 of such a series are examined, one notices that the first sections which 

 pass through the dorsal or neural limit of the spinal cord, contain 

 a large number of rather large ganglion cells, and it can easily be 

 verified that the roof of the spinal cord along its whole length is com- 

 posed of similar cells, which form a double row reaching from the 

 termination of the hind- brain to the posterior limit of the central 

 nervous system. 



Their occurrence in the brain region proper is very doubtful ; as 

 they appear to extend forwards only as far as the anterior boundary 

 of the hypoglossus region. That is, in early stages they can be traced 

 as far as those somites which belong to the hypoglossus. 



They are the first cells in the embryo which develop ganglionic 

 characters, and they are fully developed in young embryos long before 

 the remaining cells of the nervous system become ganglionic. The 

 cells are multipolar ; and in some cases processes can be seen passing 

 from them into the developing spinal cord, but I cannot say how they 

 are connected with other nervous elements. Possibly they are paired ; 

 at any rate, in many cases they are bilaterally arranged, and in the 

 region of each mesoblastic somite in the different fishes to be presently 

 mentioned from four to eight pairs of such cells occur. The exact 

 number can only be determined by means of reconstructions of 

 embryos. There is probably a defined number of them in every 

 embryo of each species, and this number must be several hundred. 



The most remarkable circumstance is their fate, which I have so 

 far fully determined in Scyllium, Pristiurus, Lepidosteus, Salmo, and 

 Triton. 



On the Formation of the Permanent Central Canal of the Spinal Cord. 



These Ganglion Cells are all shut out of the Central Nervous System. 

 Their processes are either withdrawn or cut off, more probably the 

 latter, and their poles now present a curious stumpy appearance. The 



