No. 408.] STUDY OF MAMMALIAN EMBRYOLOGY. 923 
spinal cord is clearly divided into a dorsal (D.Z.) and a ventral 
zone ( V.Z.) on each side; the two dorsal zones are connected 
bral ; 
inferior maxillary nerve ; Of., optic nerve; PA., pharynx; Ret.,retina; V.R. 
tral zone of spinal cord; 7, facial nerve; 9, glossopharyngeal 
jams. 
ganglion; 70.77, vagus accessorius nerve. x 22 
across the median line by the thin deck plate, and the ventral 
zones similarly by the thin floor plate. The lower or ventral 
limit of the dorsal zone is marked externally by the entrance of 
