No. 6.] AMERICAN MORPHOLOGICAL SOCIETY. 353 



through the substance of the epithelial cells, which with the 

 continued growth of new fibres become more and more honey- 

 combed. Their outer zone is finally transformed into a fibrous 

 framework, the " Randschlcier" which accordingly owes its 

 structure to the activity of the growing nerve fibres, and is 

 not pre-formed. 



The dorsal cells or giant cells of Rohon arise in the clorso- 

 lateral portion of the cord next the outer limiting membrane. 

 They elongate, and for the most part each cell gives rise to an 

 ascending and a descending nerve fibre forming the beginning 

 of the dorsal funiculi. Gradually the cells leave their origi- 

 nal position and wander to the dorsal mid-line of the cord. 

 Through this movement the cells become unipolar, remaining 

 connected with their fibres by a slender process, which divides 

 in T-fashion at the point where the longitudinal fibres begin. 

 A large number of the cells form peripheral nerves also, which 

 are segmentally arranged. The dorsal cells are homologous 

 with the " Hintersellen " found in Petromyzon, and with the 

 bipolar cells of medium size in the cord of Amphioxus. They 

 are to be regarded as a primitive type of sensory cell identical 

 in function with the spinal ganglion cells, with which they are 

 genetically related. 



XIX. THE SPERMATIC AND MESENTERIC 



ARTERIES OF DIDELPHYS VIRGINIANA 



(KERR, LINN.). 



C. F. W. McCLURE. 



IN mammals other than -marsupials the anterior mesenteric 

 artery supplies the small intestine and the proximal end of the 

 large intestine. The posterior mesenteric artery is given off 

 from the posterior division of the abdominal aorta, and supplies 

 the large intestine. In a large number of mammals the internal 

 spermatic arteries are given off from the aorta about midway 

 between the renal and posterior mesenteric arteries. 



In Didelphys and other marsupials, so far as known to the 

 writer, the anterior mesenteric artery supplies both the small and 



