82 PLATT. [Vol. V. 



they have undoubtedly the morphological value of spaces cut off 

 from the primitive ccelom. The position, the independence, 

 and the time of origin of the anterior pair of cavities, tend to 

 show that these cavities have a segmental value equal to that of 

 the lateral parts of the premandibular cavity. Van Wijhe (No. 

 16) found in Galeus a pair of head cavities anterior to the pre- 

 mandibular, and thinking them recently cut off from the preman- 

 dibular cavity, homologized them with prolongations of that 

 cavity which he had seen in Scyllium. Denying to them, there- 

 fore, a value equal to that assigned by him to the lateral por- 

 tions of the premandibular cavity, he considered them, and their 

 homologues in Scyllium, to represent the visceral portions of 

 the premandibular cavity. It may be mentioned here that the 

 median portion of the premandibular cavity is not ventral to 

 the lateral prolongations of that cavity, as claimed by Marshall 

 for Scyllium, but dorsal, as more recently shown by Dohrn in 

 his studies on Torpedo (No. 8). 



Longitudinal sections through Acanthias embryos at a stage 

 when the anterior gill clefts are as yet but pockets from the 

 alimentary canal,' show serial divisions of the mesoderm that 

 differ somewhat from the divisions in Scyllium described and 

 figured by Van Wijhe (No. 16). I shall merely note the differ- 

 ences here, hoping later to give the subject more careful study. 

 From the anterior limit of the head to the tail the neural cord 

 is marked, at this stage, by a series of constrictions, opposite 

 each of which there lies, in the body, a mesodermic somite, or 

 protovertebra. The line of somites alternating with the neuro- 

 meres is continued into the head as far forwards as the ali- 

 mentary pocket, which is to form the second visceral cleft. 

 Here complete divisions of the mesoderm cease, but serial 

 depressions in its dorsal wall indicate incomplete divisions into 

 three parts above the hyoid arch (Van Wijhe found two somites 

 here) and two parts above the mandibular arch. Like the 

 somites of the trunk, the divisions thus marked off alternate 

 with the neuromeres, lying opposite successive constrictions of 

 the brain. The anterior division of the mandibular cavity cor- 

 responds to the constriction that separates the mid-brain from 

 the hind-brain, or to that from which the trochlear nerve arises. 



In Fig. i, PI. IV, the four anterior cavities of the right side 

 of the head are represented at a stage when the hyomandibular 



