Hamitton, Dzvzston of Cells in Nervous System. 301 
This question as to the situation of the dividing cells and 
incidentally, the period up to which they persist in the central 
nervous system has been dealt with by ALTMANN (1881), Rav- 
BER (1881-82-86), MeERK (1886), His (1886), VicGNnaL (1889), 
SCHAPER (1897) and by Paton (1900). 
ALTMANN (Z) was the first to point out the ventricular situ- 
ation of the dividing cells and laid down the rule that in all 
organs of epithelial origin, cell division takes place on one sur- 
face only, that farthest removed from the mesoderm. Hits (2) 
confirmed this statement in his study of the human embryo of 
one month. RavuBER (3), on the contrary, denied any ‘‘seat 
of predilection” for the dividing cells. In his earlier publica- 
tions he takes a more decided stand than in his later, but in all 
he insists that no layer of cells is exclusively the seat of cell 
division, and that the distribution of mitotic figures varies greatly, 
predominating now in the innermost layer, now in the outer 
layers. At one stage, for instance, in the spinal cord, the 
largest number is found in the anterior horns of the gray mat- 
ter. His researches on frog embryos of 4.6 mm. and of 15 mm. 
show that at both these stages the ventricular mitoses predom- 
inate, but extra-ventricular ones always are present. He ex- 
plains the apparently contradictory views held by different ob- 
servers on the ground that the material used represented very 
different stages of development. 
Merk (4) worked on embryos of fish, amphibia, birds and 
mammals, including rats). He upholds ALTMANN’sS view very 
strongly, and although he admits that scattered mitotic figures 
are in some instances found, yet those situated on the ventric- 
ular surfaces are always—in the cord and cerebral hemispheres 
—very much inthe majority. An exception is found in the 
early stages of development in fish and amphibia, where for a 
short time, cell division is equally distributed in all layers.’ But 
in the higher vertebrates from the time of the closure of the 
1 It is remarkable that MERK found the diffuse arrangement of the mitoses 
in the early stages, while most observers, as will be seen, find this, if at all, 
in the later stages. ; 
