232 ALBERT KUNTZ 



vision and give rise to new generations of "indifferent" cells 

 which may develop either into neuroblasts or into embryonic 

 supporting cells. The accompanying figure (fig. 9), modified 

 from Schaper, has been introduced to illustrate the developmental 

 relationships of the neurones and the supporting elements in the 

 embryonic nervous system. According to Schaper's original 

 description, the "indifferent" cells are characterized by large 

 rounded nuclei showing a delicate chromatin structure, and very 

 little cytoplasm. The "neuroblasts" are characterized by large 

 rounded nuclei showing little structure in the interior except a 

 well defined nucleolus, and a larger cytoplasmic body which is 

 early drawn out to a point at one side. 



The great majority of the cells migrating from the neural tube 

 and the spinal ganglia, along the spinal nerves and the communi- 

 cating rami in embryos of the pig, answer to the description of 

 the "indifferent" cells of Schaper. When observed in the motor 

 niduli or in the distal ends of the spinal ganglia, their nuclei usu- 

 ally appear more or less rounded in outline and show a very 

 delicate chromatin structure. The cytoplasm is so meager that 

 it can be observed only under the most favorable conditions. As 

 these cells migrate peripherally they assume a more elongated 

 form. In the ventral nerve-roots many of them have assumed 

 their maximum elongation soon after they have left the neural 

 tube. In the lines of cells which may be observed migrating 

 out of the neural tube, the inner ones are often nearly circular 

 in outline, the outer ones are greatly elongated, while those in in- 

 termediate positions show varying degrees of elongation. This 

 elongation cannot be accounted for mechanically, as by the 

 squeezing through a narrow aperture in the external limiting- 

 membrane. These apertures are usually broad enough to per- 

 mit free passage of the cells. It is probably due to more subtle 

 forces which are operative in the process of migration. Further- 

 more, it is by no means certain that such a change of shape al- 

 ways takes place. Rounded "indifferent" cells are sometimes 

 observed far distally along the course of migration, while elon- 

 gated cells are present in the motor niduli and in the distal ends 

 of the spinal ganglia. 



