Irving HarJesty 241 



Altman, 8i, was, I think, the first to call attention to the locality in 

 which the mitoses occur. He thought that all divisions take place within 

 the zone immediately bordering the ventricle — the ependymal layer. 

 This view was supported by His, 87, as true for human and rabbit 

 embryos. Eauber, 82, using a series of frog embryos, denied a special 

 seat of cell-division, having found mitotic nuclei in both the ependymal 

 and outer layers. He explained the contradiction as due to others having 

 taken material at different stages of development, admitting that in tlie 

 very early stages all mitoses are ventricular. Merk, 86, and Vignal, 89, 

 admit that cell-division may occur in other regions than the ependymal 

 layer. The latter, hawever, did not actually observe any elsewhere but 

 assumed their occurrence from a second assumption that the nuclei 

 increase more rapidly than could be accounted for by the ventricular 

 mitoses observed. Schaper, 97, in his studies of the early differential 

 processes in the embryonic nervous systems of various species, touches 

 upon this point and reaches a conclusion which, from my study of pig 

 material, I am convinced is correct. He observed that it is only in 

 very young embryos that all the mitoses occur in the ependymal layer; 

 that after the blood-vessels have entered the central system and begin to 

 elaborate, extra-ependymal mitoses may be found, and that therefore 

 the distribution of mitotic nuclei is largely a question of proximity to 

 nourishment. In the early stages, the ependymal zone may also be 

 considered a locality presenting less mechanical resistance to the phenom- 

 enon. Hamilton, 01, quotes Schaper and practically accepts his conclu- 

 sions as true for the white rat. Paton, 00, investigating the histogenesis 

 of the cerebral cortex of the pig, finds most of the dividing cells in the 

 ependymal layer and very few in the mantle layer. In my sections of the 

 spinal cords of pig embryos nearly all the extra-ependymal mitoses are 

 found in the middle nucleated layer, which itself results from the migra- 

 tion of nuclei from the ependymal layer, and which is distinguished, on 

 the one hand, by having its nuclei less thick than in the ependymal layer, 

 and on the other, by the fact that the adjacent mantle layer proper is 

 relatively free from nucl-ei (see Figs. 5 to 8). While the middle layer 

 begins to appear quite early, it never presents mitotic nuclei till some- 

 time flfter the blood capillaries are acquired. 



The following observations may convey some idea of the rate at which 

 division occurs in the spinal cord of pig embryos and the relative num- 

 bers in the ependymal and extra-ependymal layers : 



At 5 millimeters (unflexed) all mitoses are not only in the epend5T3ial 

 layer but in the ventricular surface of that layer. Counts of twenty 

 sections of oix each give an average of 5 mitoses per section. 



