302 THE DEVELOPMENT OF THE VENTRAL NERVES IN SELACHII. 



von Kolliker, von Lenhossek, Ramon y Cajal, Retzius, and others. In support 

 of it the following facts are urged: First, in adult ventral nerves the neuraxones 

 are demonstrably (most convincingly by the Golgi method) processes from medullary 

 cells. In embryonic amniotes ventral nerves in their early stages are purely fibrillar 

 structures to which cells are secondarily added. These fibres are seen to be processes 

 of medullary cells. By the Golgi method various stages in the elongation of the 

 neuraxon may be demonstrated. In the fibre tracts of the brain and spinal cord 

 are found in regions devoid of cells extremely elongated neuraxones which must, 

 therefore, be processes of a single cell. A similar extension of neuraxones in the 

 peripheral nervous system is consequently not improbable. Finally, experiments 

 upon the regeneration of neuraxones strongly support the conclusion that these 

 structures are processes of a single cell. 



A third hypothesis of nerve histogenesis has been advanced by Sedgwick ('94), 

 who regards nerves as local differentiation of a cytoplasmic reticulum connecting 

 cell with cell. Sedgwick regards the embryo as a protoplasmic syncytium and the 

 nerves as specialized strands of the protoplasmic network. Too little constructive 

 work has been done in the study of nerve histogenesis from this point of view to give 

 this hypothesis a standing comparable with that of the other two stated above. The 

 problem of ventral nerve histogenesis in Squalus is therefore stated Are neuraxones 

 formed by the fusion of cell chains or by cytoplasmic outgrowths from medullary 

 cells? 



6. Are the cells which form the neurilemma emigrated medullary elements (ecto- 

 dermal) or mesenchymatous (mesodermal) cells? Many embryologists, Balfour 

 ('81), Dohrn ('88, '91, '91 a ), Beard ('92), Van Wijhe ('89), Beraneck ('87), Miss Platt 

 ('91), Kupffer ('94, '95), Hoffmann ('97), and Raffaele (:00), have shown beyond a 

 doubt that embryonic nerves are cellular. The origin of the cells is in doubt. Dohrn 

 ('88, '91) gave strong evidence that in Selachii they are emigrated medullary elements. 

 He found that not only are the nuclei of ventral nerves larger than those of the mesen- 

 chyme, but that similar nuclei lie nearer the medullary wall while other similar ones 

 lie half in and half out of the wall, and in later stages a greater number of such cells 

 are seen apparently in the very act of emerging. In a later paper ('91 a ) he regarded 

 this conclusion as more than doubtful for spinal nerves, but still maintained its truth 

 for the oculomotor. Van Wijhe ('89) found the ventral nerves cellular in selachian 

 embryos, but with characteristic cautiousness of statement gives no opinion concern- 

 ing the origin of the cells. His ('89), von Kolliker ('92, :00) and von Lenhossek ('97) 

 have assumed them without sufficient evidence to be mesenchymatous origin in 

 Selachii. Von Kupffer ('94, '95) found in Petromyzon and Acipenser a marked simi- 



