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). 
Ill. Tur 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 
