NEAL: NEKVOUS SYSTEM IX SQUALUS ACANTHIAS. 199 



from the ventral to form the muse, obliquus superior, can by virtue of its 

 topogi'aphic relations to chorda and aorta be regarded as the somatic 

 portion of this mesoderm segment (Plate 7, Fig. 56). Its ventral portion, 

 which later becomes differentiated into the muse, adductor mandibulse, 

 is therefore splanchnic. While the indications of the differentiation of 

 the 2d somite into myotome and sclerotome are less clearly expressed 

 than in the case of the 3d and ith, I have no reason to question the 

 correctness of Killian's ('91) interpretation that such appear. The 

 great enlargement of the cavity of the somite is the chief factor in 

 modifying its form and the relations of its constituent parts. While 

 Miss Piatt ('91) finds the musculature to arise first in the median wall 

 of the somite, that is to say, the dorsal -part of the so called " mandibular 

 cavity," Hoffmann ("96) states that the muse, obliquus superior arises in 

 its upper and lateral walls. In my opinion their conclusions are not so 

 divergent as they might at first sight seem to be, for I believe that the 

 portion of the somite which Hoffmann calls dorsal is morphologically 

 viedian ; in other words, that it is the portion which in early stages lies 

 against the \\all of the neural tube (Plate 7, Fig. 56). I agree with 

 Hoffmann that the muse, obliquus superior arises in the dorsal and lateral 

 walls of the second (van Wijhe's) somite, but with the qualification that 

 the dorsal wall is morphologically median.^ 



The first (premandibular) somite shows in its development even greater 

 peculiarities than those of the mandibular ; yet it appears to me to pos- 

 sess somatic value as unquestionably as the latter does. The first and 

 most important question to answer is whether this segment represents 

 dorsal mesoderm or a diverticulum from the alimentary canal, and for 

 this purpose the relations of the connecting stalk furnish us with the 

 decisive evidence. In a median sagittal section of an embryo with 

 II or 15 somites, such as that shown in Figure C, the tissue which is later 

 difterentiated as the connecting stalk of the first somite appears as a 

 mass of cells between the base of the brain, in that region which lies 

 just posterior to the pit of the infundibulum, and the dorsal wall of the 

 alimentary canal. Posteriorly this mass of cells is continued into the 

 chorda and its relations are seen to be such that, if the chorda is dorsal, 

 80 must the mass of cells be also. The lumen of the alimentary canal 

 may be traced to a point directly ventral to the pit of the infundibulum, 

 where it ends as the so called '' Seessel'sche Tasche " (Knpffer's " pra- 



1 Miss Piatt's ('91") evidence of the continuity of the cavity of the alimentary 

 canal and that of the mandibular cavity, as well as her evidence of two segments 

 in the latter, appears to me illusory. 



