Houser, The Neurones of a Selachian. 155 
trate to deeper levels. The dendritic surface bears small gem- 
mules and larger bosses in considerable numbers. 
The axone is a direct continuation of one of the dendrites. 
Its point of origin is indicated by the surface becoming smooth 
and the course more irregular. The axone continues to hold a 
course tangential to the limitans externa, and it terminates 
through a few small branches at no great distance from its point 
of origin. This type of neurone is evidently purely associative, 
linking together areas not widely separated. 
Structurally, the CayaL neurone is somewhat peculiar (Fig. 
69). The tigroid substance is collected into a few lumps of 
relatively large size. These are disposed irregularly in the 
great masses of cytoplasm which lie lateral to the nucleus. The 
nucleus is eccentric in position, leaving but a thin pellicle of 
cytoplasm on one of its sides. The chromatin is grouped into 
a few strands having conspicuously thickened nodes at intervals. 
a. General Considerations on the Pallium.—The pallium of 
the selachian is really a primitive representative of the stem 
giving origin to the pallia of amphibians, reptiles, birds, and 
mammals. The continuity of its two halves, contrasted with 
the two-lobed condition of the higher type, has caused many 
writers to rank the selachian pallium asa divergent branch. 
The correctness of the view set forth in this paper will appear 
when the significance of the relations which exist between cer- 
tain neurones of Mustelus have been considered. 
It has been shown that the ventral border of a lateral ven- 
tricle is the seat of one olfactory termination, the epistriatum. 
The epistriatum tends to become extended round the side of 
the ventricle toward the pallium. Axones from the marginal 
neurones of this olfactory centre enter the pallium, thus serving 
to link that region indirectly with the olfactory organ. Besides 
this source of impressions, the olfactory centre of the nucleus 
neuroporicus radiates an influence to the pallium, of which it 
is really a part. The pallium of Mustelus, therefore, is seen 
to have a general association with the olfactory mechanism. 
It has been shown by EprncEr (’96) that the special pal- 
lial olfactory conducting path characteristic of higher verte- 
