204 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 



the origin of all the parts of a peripheral neuron in the following manner ; 

 The neuraxon arises as a process from a centrally situated neuroblast, 

 •which becomes the ganglion cell. Cells of mesodermal origin form 

 around the neuraxon the sheath of Schwann (\''ignal, '83 ; Kolster, '99 ; 

 Gurwitsch, :00 ; Bardeen, :03 ; and others). Between the central 

 neuraxon and the enveloping sheath of Schwann the non-cellular medul- 

 lary substance is developed, possibly as ifye result of differentiation in 

 the homogeneous stroma immediately surrounding the neuraxon. The 

 participation of emigrant medullary elements in the formation of the 

 nerves appears, then, to be quite unnecessary. 



Recent writers on the histogenesis of nerves generally take into ac- 

 count, however, the indisputable evidence that has been offered in favor 

 of the immigration of cells into the developing nerves of elasmobranchs. 

 Bardeen (:03) admits that even in mammals, as well as in the lower 

 vertebrates, a certain number of cells migrate out from the spinal 

 ganglia and cord, but believes that in mammals the cells of Schwann's 

 Bheath arise in the main at least from the mesoderm. Neal (:03) is of 

 the opinion that in elasmobranchs both mesodermal and emigrant 

 •ectodermal cells participate in the production of the sheath of Schwann. 

 He saw few mitoses among the emigrant cells, and consequently con- 

 cludes that ectodermal elements are not present in sufficient numbers to 

 furnish all the sheath cells. In the developing oculomotor and abdu- 

 cent nerves in chick embryos however, mitotic division among the " ac- 

 companying " cells is of frequent occurrence. If, as the appearances 

 Jead me to believe, the " accompanying '' cells are medullary derivatives, 

 it follows that here ectodermal elements might easily be multiplied until 

 they became numerically equal to the nuclei of the Schwann's sheaths 

 of the definitive nerve. Proof that the Schwann's-sheath cells of the 

 lateral line in Amblystoma are to be regarded as ectodermal derivatives 

 has been adduced by Harrison (:03) in a recent paper. 



In an earlier paper Neal (:00) concluded from his own researches, 

 and from a review -of the literature, that anamniote embryos differ from 

 amniote embryos in deriving a part of their sheath cells from the neural 

 tube. That migration is more easily observable in lower forms than in 

 mammals is undoubtedly true. In this connection it should be borne 

 in mind that His ('89), who controverted the interpretations of Dohrn 

 ('88°) in respect to migration in selachians, must have been largely influ- 

 enced by his extended researches on the histogenesis of mammalian 

 nerves. But if it be admitted that medullary cells migrate out of the 

 neural tube into the roots of the third and sixth nerves in the chick, 



