JOHN J. MASON. 3 



related extremities, and has since been repeatedly con- 

 firmed in frogs, ■^ especiall)^ in longitudinal sections. 



The caudal region in turtles and in those lizards which 

 have few and delicate caudal muscles furnishes an inter- 

 esting fact for consideration. In turtles the cell nuclei 

 gradually diminish in size from before backward, and 

 finally disappear altogether near the posterior portion, 

 where the horns of gray matter present much the same 

 appearance, as to structure, as that of the same parts in 

 the dorsal region. 



While in the alligator some of the largest cell nuclei are 

 met with in this part of the cord, in those saurians, helo- 

 derma especially, which have comparatively little power in 

 the tail, these elements are reduced gradually in size in the 

 same sense as are those in the turtle. The same gradual 

 transition is well marked in the caudal region of Scincus 

 Erythrocephalus. 



Stieda* gives measurements of nerve cells and their 

 nuclei from the various parts of the spinal cord in Testudo 

 Graeca and Emys Europaea, agreeing with my own made 

 later, and concludes as follows : 



" I guard myself expressly against the supposition that 

 the great differences in size between these three (large, 

 medium-sized, and small) classes of cells are evidences of 

 different physiological importance in these elements. I 

 wish rather to assert that what is found in the spinal cord 

 of the turtle can and must be used to support the contrary 

 view. The fact that in the caudal and dorsal regions no 

 large cells exist, but only medium-sized and small cells, while 

 inferior (motor) roots are given out from these same regions, 



* In the spinal cord of a bat which I have lately examined, the nuclei of the 

 cervical region were found to be far more abundant than those of the lumbar 

 region, and their average diameter somewhat greater. The muscles of the 

 two pair of extremities bear the same sort of relation to each other. 



" Ueber den bau des centralen nerven systems der amphibien und rep- 

 tilien." Axolotl and Schildkrote, Leipzig, 1875, p. 40. 



