242 The Development of the IS'euroglia 



At 7 millimeters (unfiexed) mitoses are more abundant, but all are 

 in the ependymal layer. They are more numerous in the ventral than 

 in the dorsal portion of the section and often occur embedded in the 

 layer, not touching its ventricular border. Counts in twenty sections of 

 5/x give an average of IG mitoses per section. 



At 10 millimeters dividing nuclei are still more numero^^s. Now and 

 then evidence of blood capillaries may be seen, and, very occasionally, 

 an extra-ependymal mitosis. Average mitoses in twenty sections, 24 per 

 section. 



In the spinal cords of pigs of 13, 15, 17 and 20 millimeters, a com- 

 putation gave an average of about 20 mitoses per section. Of these 

 about one dividing nucleus per section occurred in the middle nu- 

 cleated layer. Blood capillaries increase rapidly in abundance with the 

 size of the specimen and occasional mitoses may be observed in their 

 walls. These were not taken into account. 



At 25 millimeters the number of mitoses begins to decrease appre- 

 ciably, the decrease occurring entirely in the ependymal layer, and in 

 pigs of 35 millimeters mitoses in this layer are but seldom observed. 

 From 4 centimeters on there are practically no divisions to be found 

 save what is apparently either a leucocyte or at least a nucleus of meso- 

 dermal origin. 



In pigs of 25 millimeters, transverse sections of the spinal cord begin 

 to suggest the characteristic shape of the adult. The more rapid lateral 

 growth begins to result in the ventral and dorsal median fissures, and the 

 nuclei begin to be so arranged that the form of the gray figure may be 

 distinguished. At 30 millimeters the central canal, which hitherto has 

 maintained the relative proportions of a ventricle, suffers a collapse of 

 its dorsal two-thirds and the remaining, ventral third begins to assume 

 the circular form (compare Fig. 8 with Fig. 6) and the ependyma to 

 appear under low power as a single layer of ciliated epithelium. Merk, 

 86, makes the statement that in both birds and mammals nuclear division 

 ceases when the central canal has become circular^ and the ependyma 

 ciliated. I find that in the pig, however, evidences of cilia appearing 

 out of the internal limiting membrane as early as 10 millimeters. 



The above observations agree in the main with what others have found 

 in embryos of other species, namely, that karyokinetic activity increases in 

 the earliest stages, reaches a period of maximum activity which is main- 

 tained for a time, and then declines, and finally, at a comparatively early 

 stage, the division of nuclei of ectodermal origin ceases altogether. 

 Both the period of maximum activity and the time of cessation varies 

 for different animals. Hamilton, oi, thinks that in the brain of the rat 



