JOHN J. MASON. 5 



cles of the lower jaw and next those of the centres of the 

 oculomotorius. This is true of all four species. 



In the first three animals weighing about four pounds 

 each, the nuclei for the respective centres were about equal, 

 while differing in size in each individual as stated above. 

 In the Chelydra Serpentina (snapping turtle weighing 24 j^ 

 pounds) all the motor nuclei were much larger than those 

 of the smaller specimens. The same rule holds true in 

 frogs and alligators. The smaller the animal, the smaller 

 the cell nuclei. I have not seen any mention of this fact 

 in any works on anatomy. 



2. The nervous centres of the alligator are especially 

 well fitted for sections, and I have obtained three series of 

 preparations, many hundred in all, showing the nuclei of 

 the cells of origin of all the motor nerves. In this animal, 

 the cell nuclei of the motor root of the trigeminus are found 

 to occupy, as to size, the same middle rank between the 

 nuclei of the oculomotorius and those of the motor roots 

 of the spinal nerves, that they do in the turtle. These nu- 

 clei are remarkably large in both the alligator and snapping 

 turtle. 



3. In frogs the rule is even more strikingly illustrated 

 than in the animals just mentioned. Anatomists have, as 

 yet, made no centre for the hypoglossus, but it is interest- 

 ing to note that, the nuclei of the cells forming the " nu- 

 cleus centralis " of Steida, described and figured also by 

 Reissner (the natural centre for this nerve), have a diameter 

 just between that of the nuclei related to the oculomotorius 

 and those related to the motor root of the trigeminus. 



The constant variations in the size of these elements 

 above indicated have been clearly illustrated by photog- 

 raphy. By employing exactly the same degree of enlarge- 

 ment — by using the same objective and having the same 

 distance always between the focusing screen and micro- 



