304 JOURNAL OF CoMPARATIVE NEUROLOGY. 
VIGNAL’s results are still harder to bring into harmony with 
mine, for his illustrations show all periods of develooment from 
the long narrow tube with only the germinating layer of cells, 
to the fully formed cord with definite groups of multipolar 
cells, but throughout the series he finds not one extra-ventric- 
ular mitosis. 
Although RavusBer himself suggests that the variation in 
number and position of the dividing cells is due to the period 
of development of the embryo in question, yet apparently he 
finds no difference between his frog embryo of 4 mm. and the 
one of 15 mm. 
Paton gives drawings (see his Figs. 4 and 6) from pig em- 
bryos which correspond to the brain of the foetus and of the 
new-born in my series, but he finds very few extra-ventricular 
mitoses in either; only one is represented in his drawings. Even 
SCHAPER, although he mentions the appearance of these extra- 
ventricular dividirlg cells as coincident with the development of 
the blood-vessels, apparently does not attach much importance 
to them, as he gives no description of their number, nor of the 
exact areas in which they are found. There is no mention any- 
where in the literature of dividing cells being found in the nerve 
fiber tracts. 
It might be suggested that the grade of the animal in the 
zoological series is an important factor, that in the lower verte- 
_ brates the course of development is different from that in the 
higher, and this may be true, but so far there is not much in 
the literature to confirm it. His, working on early human em- 
bryos, arrives at the same conclusions as MERK, who uses em- 
bryos of fish, amphibia, birds and mammals. 
MeErK alone makes an absolutely definite statement as to 
the period at which cells cease to divide. According to him, the 
dividing cells gradually diminish in number and disappear by 
the time the central canal has become circular, the ependymal 
cells ciliated, and the posterior median fissure formed. This 
represents a stage which the cord of the white rat has attained 
at birth. 
To recapitulate the points in which the results of this study 
