Ross Granville Harrison 123 
these experiments gave inconclusive results, as small ganglia were al- 
ways found later, showing either that their normal rudiment had not 
been entirely removed, or that they had regenerated from some other 
source. In one experiment, however, in which the embryo was preserved 
four days after the operation, an examination of the serial sections re- 
vealed no ganglia except several sporadic cells on the n. facialis and n. 
vagus. ‘These nerves consist of naked fibers, except that several sheath 
cells are present near their origin. A nerve in front of the facial, prob- 
ably the oculo-motor, but perhaps the motor part of the trigeminus * is en- 
tirely without sheath cells and the naked fibers may be traced from the 
brain to a mass of mesoderm cells in the region of the eye. The results 
“Hind Leg 
ae : ** Abdominal Muscle 
Segmental Nerve 
Fic. 2. Profile view of frog larva (Rana palustris, 12 mm. long) after 
complete resorption of yolk. The relation of the segmental nerves and the 
primary abdominal muscles are shown. 
of this experiment, therefore, confirm the first series, showing that the 
cranial nerves may develop without the aid of the sheath cells. 
But while the sheath cells are thus demonstrated not to be a necessary 
factor in the formation of the nerve, it may still be urged that they, as 
well as the ganglion cells, might normally form some of the fibers. Dur- 
ing the past year an effort was made to solve this question by studying 
the behavior of the sheath cells in the absence of processes from the 
nerve centers. The source of the motor nuclei (ventral half of the med- 
ullary cord) was removed from embryos of the same age as in the previ- 
ous experiments, leaving the dorsal part of the cord together with the 
ganglion crest intact. The object was to ascertain whether the sheath 
* Owing to the absence of most of the important landmarks on the injured 
side the exact determination of this nerve is doubtful. 
