12 H. V. NEAL 



proximity of tube and somite is so close that the absence 

 of predetermined paths for the growing nerve is not surprising. 

 Supporters of the Hensen hypothesis have greatly exaggerated 

 the mechanical difficulty of the acquisition of a connection 

 between tube and myotome in the absence of primary paths. 

 But even the most ardent supporter of the Hensen hypothesis 

 will admit the capacity of cells to throw out amoeboid processes 

 into the surrounding medium, and for distances quite as great 

 as that which separates somite and neural tube. 



As a matter of fact, protoplasmic connection between tube 

 and somite to form the anlagen of somatic motor nerves in 

 Squalus is effected in precisely that way, as was described by 

 Dohrn ('88) many years ago. The process is one of amoeboid 

 outflow, as stated by that accurate observer, arid as shown in 

 figures 4 to 7 of this paper. Beginning with the seventh somite 

 of the selachian embryo protoplasmic connection with each of 

 the successive somites of the body is effected in the same way, 

 that is, by amoeboid processes of medullary cells of the ventro- 

 lateral wall of the neural tube opposite the middle of the somites. 

 Since the successive somatic motor nerve anlagen are succes- 

 sively formed, beginning with the most anterior, all stages in the 

 establishment of these connections of nerve and muscle may be 

 seen in serial cross-sections of Squalus embryos of all stages 

 beginning with embryos of 30 somites. The process goes on in 

 the caudal region until quite late stages are reached. 



The protoplasmic connections which form the anlagen of so- 

 matic motor nerves are not formed by the differentiation of 

 plasmodesms which have existed from the beginning as the result 

 of incomplete cell division, but are formed secondarily by the 

 free outgrowth of amoeboid processes of medullary neuroblasts. 

 The process is one of protoplasmic movement, analogous to that 

 seen in the remarkable culture preparations of Harrison and to 

 the outgrowth of the neuraxons of the Rohon-Beard cells illus- 

 trated in plates 3 and 4 of this paper. Stages in the establish- 

 ment of these connections between tube and myotome and in 

 the growth of the nerve anlagen are shown in plates 1 and 2 of 

 this paper. 



