No. i.] MORPHOLOGY OF VERTEBRATE HEAD. 101 



roots of efferent and afferent nerves primarily united. If these 

 two nerves are originally mixed nerves, then no purely motor 

 nerves are to be found in the anterior part of the head until 

 the abducens is reached. In regard to this nerve, I have noth- 

 ing to add to the little already known. It arises in Acanthias 

 by two main roots, and is at first distributed not only to the 

 walls of the third head cavity, but also to the general mesoderm 

 posterior to that cavity. Few nuclei are found throughout its 

 course, and the peripheral position of these indicates that they 

 are probably mesodermic, belonging to the nerve sheath, rather 

 than to the nerve itself. 



Summary. 



i. The curved end of the notochord passes through the dor- 

 sal aorta, thus almost severing from the main vessel an anterior 

 portion, which gives rise to a pair of relatively large branches. 



2. At first the cardinal veins open into the aorta at a point 

 dorsal to the origin of the mandibular arteries. In later devel- 

 opment, as the anterior limit of the aorta recedes, these veins 

 lose their direct connection with it, retaining, however, their 

 connection with that part of the primary aorta which was almost 

 cut off by the flexure of the chorda. Hence this part of the 

 aorta, with its pair of aortic branches, becomes incorporated 

 into the venous system. 



3. Anterior to the premandibular cavity there is a pair of 

 head cavities. These cavities arise independently, and are at 

 no time continuous with the premandibular cavity. The cells 

 of their walls are so modified, in the course of development, as 

 to resemble the muscle cells forming simultaneously in the 

 walls of the adjacent head cavities. No permanent muscle 

 is formed, and the cells are later lost in the general mesoderm 

 about the eye. These cavities are probably homologues of those 

 found by Van Wijhe (No. 16) in Galeus. Their lateral position 

 leads me to think that they cannot be homologized with any of 

 the head cavities recently described by Dohrn (No. 8) in Tor- 

 pedo. 



4. In relation to the new head cavities, the position of the 

 anterior arterial branches above described (1) is similar to the 

 position of the first aortic arch, in relation to the mandibular 

 cavity, or to the position of the arteriae ophthalmicae magnas 



