224. BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
observations confirm those of Miss Platt. According to Sedgwick C94) 
the third nerve is formed directly from the neural crest as are the dorsal 
cranial nerves [?], but arises as a differentiation of the reticulum formed 
by the breaking up of the neural crest, and first makes its appearance as 
a projection of nuclei from the mesoceplalic ganglion. His observations 
thus do not essentially differ from those of Miss Platt, their conclusions 
differing chiefly by reason of difference in theoretical views as to the 
mode of nerve development. My own evidence differs quite fundamen- 
tally from that given by previous investigators, since I find that the 
nerve develops after the manner described for spinal ventral nerves in 
Selachii and other Vertebrates, as an axis-cylinder process from “ neuro- 
blast ” cells in the ventral horn of the midbrain. At the earliest stage 
in which I have been able to detect the oculomotorius the extent of its 
development and its relationships are such as are shown in Figures F 
to H, which represent sagittal sections of a Squalus embryo with 
52 somites (approximately 8 mm. long). At this stage the thalamen- 
cephalon is just becoming differentiated from the primary forebrain 
(encephalomere I), The identification of the fibrillar process as the 
oculomotorius is made easy by a comparison of its point of attachment, of 
the direction of its long axis, and of its histological appearance with those 
of an embryo with 54 somites, where the oculomotorius is already con- 
nected with the mesocephalic ganglion. Under higher powers of the 
microscope the nerve appears as a deeply staining, highly refractive 
process, clearly distinguishable by these characteristics from the granu- 
lar and faintly staining processes of the mesenchyma cells at the base 
of the midbrain. Owing to a shrinkage, which however appears in very 
few of the specimens killed by the fixing agent used (vom Rath’s fluid) 
and always most markedly in the region ventral to the midbrain, the 
mesenchyma cells and the roots of attachment of the nerve have broken 
away from the base of the brain. Since, however, similar deeply stain- 
ing processes are seen to extend from cells in the ventral horn of the 
medullary tube towards the points where the roots may be supposed to 
have once united with the wall of the brain, the inference seems war- 
ranted that the nerve is made up of the processes of these cells, The 
latter show the characteristics described by His (’89) for the neuro- 
blasts of the spinal cord, viz. a highly chromatic nucleus surrounded by 
a thin, very deeply staining protoplasmic ring, which is prolonged into 
the axis-cylinder process. The precipitation of osmium serves to render 
the processes quite opaque and easily traceable among the remaining, as 
yet undifferentiated, cells of the medullary wall, and to make it possible 
