32 H. V. NEAL 



3. What cells contribute to the genesis of protoplasmic connections 



between tube and so7nitef What cells form the first anlagen 



of spinal somatic motor nerves? 



In stages slightly later than those just described, proto- 

 plasmic processes are extended into the intercellular space between 

 tube and somite by cells of neural tube and somite. In the 

 regions in which nerve anlagen later make their appearance 

 protoplasmic movement makes its first appearance sometimes 

 from somitic cells and sometimes from cells of the neural tube 

 (figs. 2, 3 and 4). In either case the ends of the projections 

 appear connected with plasmoid films. But the first real pro- 

 toplasmic connections between tube and somite are formed 

 by the outflows of medullary cells (fig. 4). 



In subsequent stages the reciprocal movement of medullary 

 protoplasm and mesenchymatous cells (sclerotome) results in 

 the close approximation of the two sets of protrusions and in 

 exceptional cases, such as are shown in figures 8 and 9, the two 

 appear indistinguishable. Such close approximation is rare 

 and, when it occurs, appears temporary, so that the genetic 

 relations of medullary cells to the protoplasmic bridges between 

 tube and somite seem indisputable. 



Those medullary cells, which by their protoplasmic outflow, 

 effect the first protoplasmic connections between tube and somite 

 usually stain more deeply than the adjacent cells, a fact to which 

 attention was first called by His (79). The same peculiar- 

 ity distinguishes the Rohon-Beard cells during analogous staged 

 in histogenesis; that is to say, in the stages of neuraxon pro- 

 duction. These 'neuroblasts' of the somatic motor nerves 

 are bipolar in shape (figs. 4-11), and within them may be de- 

 tected, in suitable preparations, a neuro-reticulum with in- 

 tensely stained fibrils. The reticulum does not appear to be lim- 

 ited to one pole of the neuroblast cell but extends around the 

 nucleus. To its presence may be attributed the deeply stain- 

 ing properties of the neuroblasts. The fact that neurofibrillae 

 make their appearance in the extended processes of these cells 

 sufficiently evinces their neuroblastic character. 



