Herrick, Brain of the Snake. 165 



cleft corresponding while the pneumogastric gives off branches 

 to two and apparently three rudimentary clefts and also a vagus 

 branch which passes along the mesoderm to the masses of cells 

 in the vicinity of the future stomach. The neuromer lies imme- 

 diately mesad of the auditory capsule. 



In Eutaenia III we note the existence of a peculiar elevated 

 and modified area of the ectoderm which, although it might be 

 regarded as accidental, is sufficiently remarkable to warrant de- 

 scription. It lies exactly over the site of the valvula cerebelli 

 i.e. over the chiasm of the trochlearis. The area is quite con- 

 spicuous in longitudinal horizontal sections and is differentiated 

 from the adjacent parts of the ectoderm by the commingling of 

 peculiar nuclei with numerous deeply stained granules in a clear 

 stroma (haematoxylin stain.) The cells are closely massed in a 

 spheroidal body. As the sections penetrate deeper the deeply 

 colored granules accumulate and the large epidermal cells be- 

 come fewer. There is an appearance of rapid degeneration. 

 The nuclei containing the granules vary greatly in size as well 

 as in the size and number of the granules. There seems to be 

 some ill-defined connection with the brain tube at the valve, but 

 nothing very definite was observed. The observation is record- 

 ed with a view to direct attention to the possible existence of a 

 dorsal sensory organ correponding to the trochlearis or isthmus 

 segment. On the other hand, it must be observed that nothing 

 similar has been seen in younger Eutaenia embryos or in those of 

 the black snake. 



In E. Ill the third (oculo-motor) nerve arises from the 

 very distinctly expanded base of the mesencephalon just ce])ha- 

 lad of the isthmus. The cells which give rise to the fibers are 

 formed from neuroblasts from a sharply localized area and the 

 fibers pass out directly. It seems to me that the nerve is con- 

 tinually replenished by neuroblasts which pass out along the 

 roots and add to the originally small number of cells lying in the 

 course of the nerve. These neuroblasts evidently fuse with 

 their predecessors in such a way that, as each neuroblast acquires 

 a sheath, these successive portions form the Ranvier segments 

 of the sheath of Schwann. This seems to be the uniform meth- 

 od of development whether within or outside of the central 

 organ. The third nerve fibers pass ventro-cephalad and cross 



