THE DEVELOPMENT OF THE VENTRAL NERVES IN SELACHII. 299 



this. Dohrn ('91) afterwards admitted that his conclusions concerning the migration 

 of cells into the spinal ventral nerves were more than doubtful, though he still main- 

 tained that medullary cells migrate into the roots of the oculomotorius. 



B. Histogenesis of the Nidultts in Sqtjalus. When the closure of the neural 

 tube takes place in Squalus, its lateral walls are formed of undifferentiated columnar 

 epithelial cells. The cells whose processes form the neuraxones of the ventral nerve in 

 its earliest stages (PL XXII, Figs. 1-4) are in every way similar to adjacent epithelia* 

 cells. Neuroblasts and spongioblasts in Squalus are undifferentiated in the early stages 

 of ventral nerve development. A few of the neuroblasts, but not all, are clearer and 

 more transparent than the adjacent cells. In Squalus, then, the first neuroblasts 

 are not developed from rounded " germinative " cells (His) but directly from epi- 

 thelial cells indistinguishable from those of the surrounding epithelium. The forma- 

 tion of the neuroxon processes of these cells before any migration takes place is a 

 further point of difference between nerve histogenesis in Squalus and in the forms 

 studied by His ('89). 



In later stages of development (PL XXII, Figs. 9-11; PL XXIII, Figs. 12, 13) 

 the neuroblasts migrate laterad toward the developing ventral nerves and lose their 

 primary connection with the membrana limitans interna of the neural tube. During 

 migration they become pear-shaped, as described by His, and stain more deeply than 

 the adjacent epithelial cells. In the meantime, the lateral walls of the neural tube 

 which were primarily formed of a single layer of epithelial cells become many -layered 

 by the multiplication of cells lying near the lumen of the tube (PL XXII, Figs. 10, 11). 

 Proof that the mitotic (germinative) cells become directly metamorphosed into neuro- 

 blasts, or that they actively migrate between the cells lying lateral to them, is lacking 

 in Squalus. 



Regarding the extent of the lateral migration of the neuroblasts and also the 

 question whether the neuroblasts leave the neural tube to enter the roots of the nerve, 

 the following considerations are important: 



First, at no stage of development can a cell body with a neuraxon process be 

 found in a developing ventral nerve. On the other hand, such cells are found within 

 the neural tube at all stages of ventral nerve development. Because of the deeply 

 staining properties of neuroblasts in early stages of development, were such present 

 in ventral nerves as in dorsal nerves, their presence could easily be detected. 



Second, while nuclei may be found half in and half out of the neural tube in 

 early stages of ventral nerve development, and while this evidence of migration is 

 correlated with the appearance and multiplication of nuclei in the forming nerve, 

 there is no evidence that these are the nuclei of the cells which form the neuraxones. 



