carpenter: development of the oculomotor nerve. 189 



If we look upon the proliferating cells in the mesocephalic ganglion 

 as comparable, to a certain extent, with the germinative cells of the 

 neural tube, and consider them, as Schaper has proved for the germi- 

 native cells, the producers of a generation of indifferent cells capable of 

 becoming in part nervous, in part supporting elements, then we can 

 easily account for the presence in the ganglion of many small, rounded 

 cells, almost destitute of cytoplasm. It seems probable that these 

 correspond to the indifferent cells of the neural tube. While some of 

 them may change to ganglion cells, others may develop into the small, 

 somewhat elliptical cells found in the ganglion, especially near its distal 

 end, where they pass by an easy gradation into the more elongate cells 

 lying among the fibrils of the nerve which here takes its origin. This 

 leads to the supposition that the "accompanying" cells of the nerve, 

 which later subserve a supporting function by developing the sheaths of 

 Schwann, have been derived through migration from the ganglion. 

 In support of this view I have introduced a drawing (Plate 2, Fig. 6) 

 made from sections of an embryo of Amblystoma punctatum. Group A 

 is taken from the central part of the Gasserian ganglion. It will be 

 noticed that here, lying within the limits of the ganglion, are to be 

 found nuclei in all stages of transition between the rounded and the 

 elongated forms. The nuclei of group B lie on the neuraxons at the 

 proximal end of the ophthalmic branch of the trigeminal nerve, those 

 designated by cl. comit.' and cl. comit." being situated at the emergence 

 of the nerve from the ganglion. The oval one (cl. comit.') I take to 

 represent a stage in the differentiation of a rounded cell into a long 

 " accompanying " cell, such as those to be found in the remainder of the 

 course of the nerve. Neither in the chick nor in Amblystoma is there 

 at any point along the ophthalmic branch evidence of an intrusion of 

 mesodermal cells. 



It is quite possible that supporting derivatives of indifferent cells 

 remain in the ganglia, and later, through their activities, form the 

 nucleated capsules of the ganglion cells, just as the "accompanying" 

 cells of the peripheral nerve form, in an analogous manner, the sheaths 

 of Schwann about the neuraxons. The similar origin of the capsule of 

 the ganglion cell and of the Schwann's sheath of its process would 

 account for the continuity of the two structures. Those investigators 

 who affirm the mesodermal derivation of the Schwann's-sheath cells 

 have never been able, so far as I know, to obtain evidence of an invasion 

 of cerebro-spinal ganglia by mesodermal elements destined to give rise 

 to the envelopes of the ganglion cells. 



