136 Journal of Comparative Neurology. 



muscle nerves seem to be derived from one common band of 

 ventricular gray on either side the median line. In the am- 

 phibia the primitive relations are little altered. The oculomotor- 

 trochlearis nidulus is very large and lies very near the ventral 

 aspect driving the dorsal longitudinal fasciculus almost into con- 

 tact with the ventral surface. The cells resemble those of rep- 

 tiles closely. The third nerve emerges a little distance caudad 

 of the mammillaria and lie in the groove laterad of the tuber. 

 Between the niduli of these nerves and the ventricle is a consid- 

 erable mass of ventricular gray of the ordinary sort with small 

 cells consisting almost wholly of the spherical nuclei. 



There are two clusters of smaller cells lying laterad of the 

 oculomotor tracts which seem to correspond with the peduncu- 

 lar niduli of other groups. Caudad of the trochlearis nidulus 

 the dorsal longitudinal fascicle turns rather abruptly dorsad and 

 soon attains its usual position along the floor of the ventricle. 



In conclusion it remains to summarize a few general results 

 of these studies hitherto. 



1. The theory of neurons is clearly seen to be the sole ad- 

 equate means of construing the concordant facts of comparative 

 anatomy and histology and of embryology. All the nervous 

 elements of the central nervous system arise as neuroblasts from 

 the ventricle and undergo more or less extensive modification. 



2. There is great difference in the degree of modification 

 of the neurons. They may become functional when in an al- 

 most embryonic condition morphologically. 



3. A considerable but variable number of the neurons are 

 held back in their development to form reserves or accessory 

 structures. 



4. The nerve tube is originally similar in the arrangement 

 of the neurons from end to end. In the Urodela this simplicity 

 is largely retained. Their brain warrants us in the belief that 

 they are permanent larvce. They are not to be regarded as illus- 

 trations of the precursor of amphibia but as a form of modifica- 

 tion which is due to the persistence of a larval state. They 

 must be used in phylogenetic speculation with considerable cau- 

 tion and only in a general way. They cannot be fairly expected 



