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 in the dorsal nerve of this segment is correlated 

 with the want of splanchnic musculature.^ That structure which I, in 

 agreement with Miss Piatt ('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 ware, mtisste man bei demselbeu das 

 ursprtingliche Vorhandensein eines Somiten und einer zugehorigen ven- 

 tralen Nervenwurzel annehmen. Yon beiden ist keiue 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 Lippenknorpel " 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' op 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. Furbringer, '97, p. 643, slightly modified). 



While Hatschek ('92), Willey ('94), and M. Furbringer ('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. 



