carpenter: development of the oculomotor nerve. 191 



lated at one side of the rounded nucleus. But an evident and con- 

 sistent difference exists between the cells taken from the two sources. 

 The cells of the ciliary ganglion are smaller than those of the ophthalmic 

 branch of the trigeminus. This difference is to be seen at once in the 

 drawings in Figure 16, in which the outlines of the cells were made 

 with the aid of the camera lucida under precisely the same optical con- 

 ditions for all groups. Not only is the amount of cytoplasm less in the 

 ciliary-ganglion cells, but their nuclei are distinctly smaller than those 

 of the ophthalmic cells. It is not an easy matter to compare accurately 

 the two classes of cells by measurements of tlie diameters of their 

 respective nuclei, since these are seldom exactly circular in outline. I 

 have, however, made measurements in the cases of such nuclei as 

 approached nearest to a circular form, and I find that while the diam- 

 eters of the nuclei of the ciliary-ganglion cells fall between 5.2 micra 

 and 6.5 micra, the nuclei of the ophthalmic cells show constantly a 

 diameter of approximately 7.8 micra. 



The ganglion cells lying in the mesenchyme, between the ophthalmic 

 branch and the ciliary ganglion, are very evidently emigrant ophthalmic 

 cells. Two such cells, having the positions y and S in Figure 13, are 

 shown in B and C, Figure 16, each surrounded by mesodermal cells. 

 A glance shows that they belong to the ophthalmic and not to the 

 ciliary type. Within the boundaries of the ciliary ganglion, lying close 

 to the exterior of the cell mass, on the side toward the ophthalmic 

 branch of the trigeminus, are to be found among the smaller ganglion 

 cells three cells with large nuclei, two of which are shown in Figures 

 16, D, and 13 (e, in both figures). These appear to be ophthalmic 

 ganglion cells, which have traversed the mesenchyme and entered the 

 ciliary ganglion. 



With the foregoing evidence before us, let us inquire into the source 

 of the cells of the ciliary ganglion. We have seen that in the early 

 stages of the growth of the oculomotor nerve a migration of medullary 

 cells takes place from the neural tube into its root. I believe that in 

 Stage I we have the first migratory cell forcing its way through the 

 external limiting membrane of the neural tube (Plate 3, Fig. 8). In 

 succeeding stages these cells seem to be migrating in considerable num- 

 bers. The rounded nuclei of the cells appear to be almost naked, for 

 it is difficult to detect any cytoplasm surrounding them. These cells 

 are as yet neither neuroblasts nor spongioblasts, but evidently the 

 motile, indifferent cells of Schaper, i. e., they are descended from ger- 

 minative cells, and are capable of differentiating later into either nervous 



