Irving Hardesty 363 



5. The ingrowth of blood-vessels into the neural tube first occurs in 

 pigs of from 9 to 10 millimeters. 



6. Prior to the ingrowth of blood-vessels, all nuclear division occurs 

 in the ventricular border of the ependymal layer and, after the ingrowth 

 of blood-vessels the great majority of the dividing nuclei and most of 

 those of undoubted ectodermal origin occur in the ependymal layer. 

 Mitosis increases from the earliest stages, reaches and maintains for a 

 time a period of maximum activity (pigs of 10 to 20 millimeters), then 

 gradually declines and, in pigs of from 30 to 40 millimeters, practically 

 ceases altogether. 



7. Throughout the lateral walls of the neural tube, the nuclei migrate 

 radially from the ependymal layer, first from the ventral half of the 

 layer, forming the anlage of the ventral horn, and then a general migra- 

 tion giving rise to a middle layer of the specimen with nuclei more loosely 

 arranged then in the ependymal layer. The mantle layer remains prac- 

 tically uninvaded by ectodermal nuclei till in pigs of 70 to 80 milli- 

 meters. 



8. The ventricle of the neural tube increases in size in pigs up to about 

 30 millimeters, then it decreases by a collapse of its dorsal two-thirds, and 

 there results a central canal but little larger than that of the adult. 

 Coincident with the collapse of the ventricle, nuclear division ceases and 

 the continued migration of the nuclei results in a thinning of the epen- 

 dymal layer, hitherto maintained quite thick, till the layer becomes an 

 ependyma similar to that of the adult. 



9. With the present technique, there is nothing to show that all the 

 products of the mitoses (germinal cells) in the ependymal layer are not 

 indifferent elements from the first— capable of developing into either 

 neurones or neuroglia. 



10. The syncytium of the neural tube, at first wholly of ectodermal 

 origin, soon becomes invaded by ingrowths of the connective-tissue syncy- 

 tium without and by two other types of mesodermal elements. The fact 

 that the syncytia from the two sources fuse before the neuroglia is formed 

 as such, and the fact that the nuclei from the two sources are similar, 

 make it difficult to determine whether mesodermal elements take part 

 in the formation of the neuroglia or not. Tissue of mesodermal origin 

 contributes appreciably to the supporting tissue of the spinal cord but, 

 by definition, such cannot be called neuroglia. 



" 11. The differentiation of the fibrils of white fibrous tissue occurs 

 considerably before the appearance of neuroglia fibers. 



13. Soon after the cessation of mitosis, the radial arrangement of the 

 syncytium of the spinal cord becomes obliterated by the further ingrowth 



