244 THE DEVELOPMENT OF THE CHICK 



a septum produced by fusion of the walls of the intermediate 

 and dorsal portions of the central canal; there is, however, a true 

 fissure produced by protrusion of the dorsal columns of white 

 matter (Fig. 146). This is, however, of relatively slight extent. 

 The original roof of the canal is therefore found between the 

 dorsal septum and the fissure. 



Neuroblasts, Commissures, and Fiber Tracts of the Cord. The 

 medullary neuroblasts may be divided into four groups: (1) The 

 first group, or motor neuroblasts, form the fibers of the ventral 

 roots of the spinal nerves. These are situated originally in the 

 ventro-lateral zone of the gray matter (Figs. 144, 145, 146): 

 they are relatively large and form a profusion of dendrites (Figs. 

 140, 141). As they increase in number and size they come to 

 form a very important component of the ventral horn of the gray 

 matter and contribute to its protrusion. (2) The second group 

 may be called the commissural neuroblasts. These are situated 

 originally mainly in the lateral and dorsal portions of the mantle 

 layer, but are scattered throughout the gray matter, and their 

 axis cylinders grow ventrally and cross over to the opposite side 

 of the cord through the floor (Figs. 139 and 140), and thus form 

 the anterior or white commissure of the cord. (3) The cells of the 

 fiber tracts are scattered throughout the gray matter, and are 

 characterized by the fact that their axis cylinders enter the white 

 matter of the same side; here they may bifurcate, furnishing 

 both an ascending and a descending branch, or may simply turn 

 in a longitudinal direction. (4) Finally there are found certain 

 neuroblasts with a short axis cylinder, ramifying in the gray 

 matter on the same side of the cord. These are found in the 

 dorsal horn of the gray matter and develop relatively late (about 

 sixteen days, Ramon y Cajal). 



III. THE DEVELOPMENT OF THE BRAIN 

 Unfortunately the later development of the brain of birds 

 has not been fully studied. The following account is therefore 

 fragmentary. It is based mainly on a dissection and sections of 

 the brain of chicks of eight days' incubation. 



Fig. 147 is a drawing of a dissection of the brain of an eight- 

 day embryo. The left half of the brain has been removed, and 

 the median wall of the right cerebral hemisphere also. The 

 details of the cut surfaces are drawn in from sections. Figs. 148 



