310 JoURNAL OF CoMPARATIVE NEUROLOGY. 
comes pointed, the cytoplasm around the nucleus forms a long 
process at the extremity of the cell which is turned toward the 
periphery. The nucleus also changes, the nuclear membrane 
becomes less thick, the chromatin masses more delicate and a 
nucleolus appears. These neuroblasts never divide, neither do 
the supporting cells which constitute the second variety of cells 
found in the neural tube. They are long, narrow cells derived 
from the epithelium of the ependyma, which elongate and send 
out a process to the inner limiting membrane and another long 
branching process to the outer limiting membrane. These 
branching and anastomosing processes form the frame-work for 
the neuroblasts in the earlier stages, later on mesoblastic ele- 
ments wander in and help to form the neuroglia,' The original 
supporting cells, which His called spongioblasts are in his opin- 
ion, incapable of division, and the increase in their number is 
not clearly explained inasmuch as he has never seen epithelial 
cells dividing to form new spongioblasts, and thinks it doubtful 
whether any of the offspring of the germinal cells become 
spongioblasts. He observed what seemed to be mitoses in 
some spongioblasts but thought such appearances should be 
looked upon with great suspicion. 
This description of the germinal cell and of the formation 
of neuroblasts is accepted by the majority of histologists, by 
VON KOLLIKER (8), by Ramon y Cajat (9), vON LENHOSSEK (70), 
Rerzius (75), Kotiman (77). The direct transformation of the 
epithelial cells of the surface of the body into germinal cells 
and of their offspring into neuroblasts, was observed by FLEx- 
NER (72) in Planaris torna. After removal of the head includ- 
ing the head ganglia the epithelial cells underwent mitotic divis- 
ion and the new cells wandered into the deeper tissues and 
passed through the stages described by His, resulting in the 
formation of neuroblasts with axones 
On the question of the formation of the neuroglia, how- 
ever, the original theory of His has received little support and 
‘ This explanation of the mesoblastic origin of some of the neuroglia has 
been withdrawn by His in his later papers. 
