244 The Development of the Neuroglia 



millimeters the embryonic connective tissue surrounding the neural tube 

 shows parallel arrangement indicative of the future paths of the dorsal 

 and ventral roots (Fig. 5). First, there appears in the ventrolateral 

 aspect of the middle layer an occasional nucleus somewhat larger than 

 its neighbors and whose chromatin shows a tendency to collect into one 

 larger mass or single nucleolus. At this time the surrounding proto- 

 plasm displays no differentiation from the general. At 20 millimeters 

 these nuclei are more abundant. Ordinary stains show no difference 

 in the protoplasm about them, but the silver method at rare intervals 

 gives pictures suggestive of the ventral-horn type of neurone. At 30 

 millimeters there is usually a group of these nuclei displayed in the 

 ventrolateral region (n. Fig. 8). With ordinary stains they begin to 

 show a differentiated cytoplasm about them and the silver method gives 

 a few characteristic nerve cells. In the anlagen of the spinal ganglia 

 the changes in the nuclei and the development of the neurone protoplasm 

 precede those in the spinal cord by at least 2 millimeters; that is, here 

 the changes begin to appear in pigs of 13 millimeters instead of 15 

 millimeters, as in the spinal cord. 



It may be said in passing, that in none of my preparations have I 

 been able to observe evidences supporting the view of either Fragnito, 

 02, or Kronthal, 02, as to the formation of the cytoplasm of the neurone. 

 After describing the changes of the nuclei into the nerve-cell type — 

 " primary nuclei " — Fragnito holds that these primary nuclei, at first 

 free from cytoplasm, become surrounded by the general smaller or 

 " secondary nuclei," which so arrange themselves and then undergo such 

 changes as to give rise to the cytoplasm and Nissl bodies of the neurone, 

 including the axone and dendrites. Kronthal ascribes a somewhat simi- 

 lar office to wandering leucocytes. 



The title of this paper bars an attempt to discuss the processes by 

 which the neurone develops. The point in mind is whether the pro- 

 ducts of the mitoses in the neural tube are not at first totally indifferent, 

 capable of developing into either neurones or neuroglia. For a long 

 time the impression was obtained, chiefly from the investigations of 

 His, that the neurone develops from the product (neuroblast) arising 

 from the mitosis (germinal cell of His) in the early ependymal layer, 

 and that the embryonic neuroglia (spongioblasts) arise as a transforma- 

 tion of the ependymal ("epithelial") cells. Contrary to this idea, in 

 1889, it was suggested by Vignal that not only the germinal cells but 

 all the cells of the neural tube are indifferent up to the time when 

 grouping is manifested in the ventral horns. Later, Schaper, 97, inves- 

 tigated the question more carefully and reached the conclusion that His* 



