292 ALBERT KUNTZ 



by large rounded or elongated nuclei usually having their chroma- 

 tin aggregated into one or two dense masses. These are obviously 

 the " indifferent " cells of Schaper. Among these are found a 

 few cells which are characterized by large rounded or elongated 

 nuclei showing one or two dense masses of chromatin, and a larger 

 cytoplasmic body which is usually drawn out to a point at one 

 side. Fig. 4 shows several of these cells drawn with the aid of the 

 camera lucida. These are obviously the " neuroblasts" of Scha- 

 per. The majority of the cells present in the mantle layer in 

 the neural tube answer to the descriptions given above for the 

 two types of cells migrating peripherally along the spinal nerves. 

 There can be no doubt, therefore, that the cells accompanying 

 the fibers of the spinal nerves have the same histogenetic rela- 

 tionships as the cells which give rise to the neurones and the neu- 

 roglia cells in the central nervous system. They are all the de- 

 scendants of the "germinal" cells (Keimzellen) of His. 



These observations are in full accord with the writer's observ- 

 ations on mammalian embryos. There is a marked difference, 

 however, in the chromatin structure of the embryonic medullary 

 cells in birds and in mammals. In mammalian embryos, the mi- 

 grant medullary cells are usually quite readily recognized by the 

 chromatin structure of their nuclei. They also usually take a 

 slightly deeper stain than the cells of the surrounding mesenchyme. 

 In birds, the chromatin structure of the embryonic medullary 

 cells differs very little from the chromatin structure of the typical 

 mesenchyme cells. Nor are they as distinctly separated from the 

 cells of the surrounding mesenchyme by differential stains as is 

 the case in mammals. Although the difficulties in technique are 

 greater in the chick than in mammalian embryos, there can be no 

 doubt that the cells accompanying the fibers of the spinal nerves 

 are migrant medullary cells. Such cells wander out of the neural 

 tube into the ventral nerve-roots in considerable numbers. The 

 number of cells present in the proximal part of the spinal nerves 

 increases rapidly until the maximum rate of migration is reached, 

 and then decreases rapidly until migration ceases, when only a 

 comparative 1 }' small, but fairly constant, number of cells remain 

 distributed along the nerve-fibers. Furthermore, a few of the 



