260 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
morphologically comparable with the hindbrain encephalomeres. It is 
in connection with a sensor nerve, the olfactory, which appears com- 
parable with the sensor portion of a dorsal segmental nerve in so far as 
it is composed of bipolar ganglion cells which send their fibres into the 
brain and, in my opinion, are in part derived from the neural crest. 
My observations upon this point, however, are as yet incomplete. ‘The 
want of motor fibres inthe dorsal nerve of this segment is correlated 
with the want of splanchnic musculature.t That structure which I, in 
agreement with Miss Platt (’91) and Hoffmann (94 and ’96), regard 
as a rudimentary somite (compare Plate 3, Figs. 16, 17, cav. a.), — since 
it resembles the following somites and gives evidence of producing rudi- 
mentary muscle cells, — represents the somatic portion of this segment. 
In correlation with the want of fully developed musculature, no ventral 
somatic nerve is present. Van Wijhe (’86°, p. 680) wrote : “Wenn der 
Olfactorius ein segmentaler Nerv wiire, miisste man bei demselben das 
ursprüngliche Vorhandensein eines Somiten und einer zugehörigen ven- 
tralen Nervenwurzel annehmen. Von beiden ist keine Spur vorhanden.” 
So far as the somite is concerned, it appears that in the “anterior so- 
mite”? we now have the requisite evidence. The lateness of the differ- 
entiation of the “premaxillar 'ippenknorpel” makes it seem at most 
only remotely possible that it may be regarded as the ventral skele- 
togenous element of this segment. 
h. COMPARISON WITH THE SEGMENTATION OF AMPHIOXUS. 
A comparison of the segmentation of Squalus as shown in Table III. 
(p. 253) with that of Amphioxus is of interest, inasmuch as it appears 
to favor important conclusions reached by the study of Squalus alone. 
However, before stating my own conclusions concerning the primitive 
metamerism of Amphioxus and the homologies of its segments with 
those of Squalus, it will be well to review the conclusions of previous 
investigators. 
A comparison of their results may be made in the form of a table on 
the opposite page (after M. Fürbringer, ’97, p. 643, slightly modified). 
While Hatschek (92), Willey (94), and M. Fürbringer (97) homol- 
ogize the mouth of Amphioxus with that of Tunicates and Craniota, but 
1 Likewise in Amphioxus the anterior dorsal nerve is generally believed to be 
purely sensor in function. 
2 Van Wijhe (’82) saw the “anterior somite ” in Galeus, but unfortunately pos- 
sessed only one embryo; he was therefore unable to express an opinion concerning 
its segmental value, based on a knowledge of its development and differentiation. 
