neal: neryous system in squalus acanthias. 195 



The evidences of irregularity in size and discontinuity in development 

 and diffei'entiation are not, in my opinion, the more serious of the objec- 

 tions raised. Such differences may indeed be explained as cojnogenetic. 

 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 dift'erentiated 

 (e. g. the eye muscle somites, 1st., 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 1st 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 1st somite of Amphioxus, makes itself manifest in the somites of 

 the more highly specialized Squalua as far posteriorly as the 7th somite of 

 van Wijhe (equivalent to the 8th somite of Amphioxus T), 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, Piatt, '04), 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 tlie 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. 



