Irving Harclesty 243 



the maximum period occurs after birth, while numerous observations 

 upon other mammals, chiefly those born much more mature than the 

 rat, are to the effect that both the maximum period and the period of 

 cessation occur long before birth. 



In the migration from the ependymal layer, the anlage of the ventral 

 horn is first to appear. It begins as an area of loosely arranged nuclei 

 on the ventrolateral aspect of the ependymal layer (vh. Fig. 5). The 

 whole ventrolateral half of the ependymal layer contributes to its forma- 

 tion. At 15 millimeters, the migration from the dorsolateral half of 

 the ependymal layer has become more active, and as a result the anlage 

 of the dorsal horn may even exceed that of the ventral in width (Fig. 6). 

 Then follows a general thinning of the ependymal layer, which the 

 hitherto abundant mitosis has maintained quite thick, till at 30 milli- 

 meters (Fig. 8), mitosis having practically ceased, the layer becomes 

 an ependyma similar to the adult form. 



The period of differentiation of the growing tissue elements into those 

 which will produce neurones and those that will take part in the forma- 

 tion of the neuroglia is difficult to determine. Certainly it may be said 

 of my preparations that in pigs up to 15 millimeters in length evidences 

 of differentiation are little more than theoretical. With the exception 

 of those nuclei in the phases of mitosis, all the nuclei of the spinal cord, 

 both in the ependymal layer and in the middle nucleated layer, are so 

 nearly identical in structure, size and general appearance that an attempt 

 to classify them on the basis of differentiation is impossible. They are 

 all of the large vesicular type, and the protoplasm about them never 

 occurs in definite form and amount, never shows definite outline, and 

 never stains differently from the general syncytium in which the nuclei 

 are embedded. Numerous nuclei, especially those in the ependymal 

 layer, show tapering masses of more densely accumulated protoplasm at 

 their either end, but these masses may be usually observed as continuous 

 with the less compact protoplasm between them. In the ependymal 

 layer these masses are portions of the radiating axes of the syncytium 

 (Figs. 4, 6 and 7). In the less densely nucleated middle layer they 

 have sometimes a stellate and often a fusiform appearance, and still 

 again the protoplasm may be present at only one end of the nucleus 

 (Figs. 6 to 9). In all cases in the early stages these appearances are 

 considered as resulting from the radial drawing out of the protoplasm 

 consequent to the later growth of the specimen and the radial direction 

 of the migration of the nuclei. 



Not till pigs of 15 millimeters do my preparations show anything in 

 the spinal cord characteristic of the neurone, although as early as 7 



