NEAL! NERVOUS SYSTEM IN SQUALUS ACANTHIAS. 195 
The evidences of irregularity in size and discontinuity in development 
and differentiation are not, in my opinion, the more serious of the objec- 
tions raised. Such differences may indeed be explained as coonogenetic, 
Rabl himself has given the evidence (’89) that the first rudimentary 
visceral cleft is differentiated later than the second. Moreover, it is 
well known that the first rudimentary myotome in Amphioxus develops 
later than the following. Differences in time of development and of dif- 
ferentiation are to be expected when a comparison is made between the 
Anlagen of serial organs, some of which become highly differentiated 
(e. g. the eye muscle somites, Ist, 2d, and 3d), while the others (e. g. the 
anterior, the 4th, and the 5th somites) are becoming rudimentary. It 
is interesting to find that the last intersomitic constrictions to be formed 
are those between the anterior and the Ist cavity, and between somites 
4 and 5, that is, the constrictions separating the most rudimentary 
somites, The separation of the anterior somite from the premandibular 
is first complete in an embryo with 26 or 27 somites, while the con- 
striction between somites 4 and 5 appears first in an embryo with 28 
somites. Consequently van Wijhe’s statement, that the segmentation 
of the dorsal mesoderm begins in the neck region and proceeds con- 
tinuously anteriorly and posteriorly, is true only in part. But it also 
follows that the discontinuity in the development of the more anterior 
constrictions may be explained as in great measure due to degeneration. 
The retardation in development due to degeneration, already apparent 
in the Ist somite of Amphioxus, makes itself manifest in the somites of 
the more highly specialized Squalus as far posteriorly as the Tth somite of 
van Wijhe (equivalent to the 8th somite of Amphioxus U), which I believe 
to be the first somite differentiated, as well as the first to develop a per- 
manent myotome.* The correlation between degeneration and retarded 
development serves to explain, for the occipital somites at least, why 
the development of the somites in the Craniota begins in the neck 
trunk. Such purely topographic relations in the Selachian cannot be regarded as 
weighty evidence in the settlement of this question, in comparison with the evi- 
dence stated for Amphioxus (van Wijhe, '93, Hatschek, 92), Bdellostoma (Price, 96"), 
and Amphibia (Houssay, ’91, Platt, '94), which has led these investigators to regard 
the visceral clefts as intersomitic in position. In view of the great probability of a 
shoving forward of the visceral clefts with reference to the somites in Squalus, I am 
unable to accept Hoffmann’s conclusion on the basis of the evidence he presents. 
1 On account of the considerable variation in the length of embryos in early 
stages of development, I am unable to state positively that the seventh somite is 
the first to develop. It may be the eighth somite which does so, as stated by Hoff- 
mann (94, ’96). The seventh somite shows some signs of degeneration, having a 
small myotome and losing its ventral nerve during development. 
