A contribution to the morphology of the vertebrate head, based on a study of Acanthias vulgaris.

THESE studies, begun under the direction of Dr. C. 0. Whitman, were completed in the summer of 1890, at the Marine Biological Laboratory of Wood's Holl.I acknowledge gratefully the valuable criticism and suggestion with which Dr. Whitnian has followed my work.My material consisted of nearly a thousand specimens, collected on the coast of Massachusetts at Lanesville and North Truro.Among the hardening reagents employed, picro-sulphuric acid proved the most reliable.However, specimens hardened in Perenyi's fluid were also good, while osmic acid and gold chloride have given some valuable results.For differentiation the common haematoxylin and carmine stains were used.The reconstructions, showing the development of the eye-No.I.] MORPHOLOGY OF VERTEBRATE HEAD.81the same stage of development.These median cavities are probably the homologues of the anterior cavities in Torpedo, recently described by Dohrn (No. 8), in which he thinks he sees the rudiments of several lost somites.I find in Acanthias little evidence to favor such a view.The median cavities lack definite walls, are irregular in number and in outline, and are surrounded by tissue which appears to be disintegrating rather than passing through a process of normal development.Sooner or later these median cavities fuse with one another and with the posterior of the paired cavities, forming together the premandibular cavity.Kastschenko (No. 9) says that the entire premandibular cavity arises L'aus der . . .praechordalen Hohle, welche einer ganzen Reihe verschiedener Umgestaltungen unterworfen wird," and that the resulting cavity cannot be homologized with a true head somite.Views so at variance as those of Kastschenko, Van Wijhe, and Dohrn show that the true significance of the premandibular cavity is still extremely problematic.For while Kastschenko claims that the cavity is non-somatic, Van Wijhe thinks it is formed by the fusion of one pair of somites, and Dohrn by the fusion of many.