14 H. V. NEAL 



has been given in an earlier paper that the cells of the nerve 

 anlagen have no genetic relations to the fibers. The fibers may 

 be traced to deeply-staining bipolar neuroblasts in the ventro- 

 lateral wall of the neural tube. 



WTien sections in metameres immediately posterior to those 

 in which protoplasmic connections between tube and somite have 

 just been established by the outflow of medullary protoplasm are 

 studied; that is, in metameres in which it may be assumed that 

 protoplasmic connections are about to be established by a simi- 

 lar outflow, before the medullary cells have thrown out amoe- 

 boid processes, deeply staining cells may be found which may 

 be inferred to be the neuroblasts of that metamere. Their more 

 deeply staining properties may be ascribed to the presence of a 

 deeply stained neuroreticulum Vithin them. Analogy with the 

 histogenesis of the Rohon-Beard cells favors this inference, since 

 within the latter, from the time of their differentiation from the 

 surrounding epithelial cells of the tube, a reticulum with a strong 

 affinity for stains makes its appearance and seems to be geneti- 

 cally related to the neurofibrillae which later make their appear- 

 ance within the long processes of these remarkable cells (plates 

 3 and 4). In Cajal preparations similar cells in similar rela- 

 tions show a deeply-stained reticulum. There is therefore rea- 

 son for thinking that the neurofibrillae of somatic motor nerve 

 anlagen have their origin in medullary neuroblasts and are not 

 exogenous in their derivation as suggested by Paton ('07). In 

 all stages the fibers of the nerve anlagen may be traced to deeply- 

 stained bipolar cells in the somatic motor column of the neural 

 tube. While it is difficult to obtain positive proof of the fact, 

 the evidence, so far as it goes, favors the opinion th^t the cells 

 which form the first protoplasmic connections between tube and 

 somite are true neuroblasts and not indifferent cells which form 

 protoplasmic paths for the nerves, as suggested by the cell-chain 

 hypothesis or by the similar hypothesis of the Hertwigs. In 

 short, connections between tube and somite are effected prima- 

 rily by neuroblasts and not by indifferent cells. These primary 

 connections appear protoplasmic and not fibrillar, only because 



