50 H. V. NEAL 



have taken for granted the mesenchymatous nature of all cells 

 associated with motor nerve anlagen. Kerr ('04) seems to have 

 made this assumption for the cells associated with the somatic 

 motor anlagen of Lepidosiren. He says (p. 121) that "richly 

 yolked masses of mesenchymatous protoplasm become aggre- 

 gated around the nerve, which till now has been quite naked." 

 Again ('02) he writes that "in tracing back the motor trunks 

 of the spinal nerves I reached a stage in which the faintly fibril- 

 lated trunk was ensheathed in a nucleated mass of protoplasm 

 of mesenchymatous origin." The mesenchymatous origin of 

 these cells has seemed so self-evident to Kerr that the problem 

 of tracing them to their source does not seem to have occurred 

 to him, and in consequence he advances not a particle of evi- 

 dence that he has traced their genesis. 



That the cells of somatic motor nerve anlagen in Squalus are 

 largely, if not exclusively, of medullar}^ derivation seems demon- 

 strated by the following facts: 



First, when cells make their earliest appearance in the nerve 

 anlagen they lie partly in and partly outside of the neural tube. 

 Then later, when cells are found definitely' within the anlagen, 

 as seen in figure 12, they appear at the base of the anlagen and 

 near the tube. As cells grow more numerous in the anlagen 

 in later stages more nuclei appear to be in the process of migra- 

 tion from the tube. As a result of the migration, the contour of 

 the tube as seen in cross-section becomes changed and the nerve 

 anlage greatly thickened. Furthermore, the nerve anlage shows 

 a limiting membrane continuous with that of the tube, which 

 makes it possible to distinguish the boundary of the nerve anlage 

 and to infer its independence of mesenchymatous cells in the 

 immediate vicinity. Moreover, the boundaries of the cells of the 

 anlagen retain their smooth epithelial contour, while the mesen- 

 chymatous cells assume characteristically irregular, branched 

 outlines. Whereas at first the cells of the sclerotome from which 

 the mesenchyma of the region is derived, are closely apposed to 

 the nerve anlagen, as seen in the figures on plates 1 and 2, they 

 soon lose contact with the nerve and with each other and become 

 metamorphosed into a loose connective tissue, but still retain 



