DEVELOPMENT OF THE NUCLEI PONTUS IN MAN 45 



closely packed young cells which pour into the pons between 

 the seventh and eighth nerves. The cells which had already 

 descended from the rhombic lip show no tendency to assume the 

 ganglionic form. They are still rather closely arranged, the cyto- 

 plasm scanty and clear, the nucleus small and containing one or 

 two chromatin condensations but no real nucleolus. Two differ- 

 ent reactions toward the haematoxylin can be made out among 

 the nuclei of these cells — the one quite densely staining and 

 usually smaller nucleus, the other slightly larger and more vesicu- 

 lar. The latter form the larger proportion of nuclei. From the 

 extensive cross-fiber system already present one must assume that 

 great numbers of these pontine cells have sent out nerve processes 

 and have taken up their final position. The newly added cells 

 tend to remain superficially yet a considerable portion push in 

 between the transverse fibers and cells already fixed. Although 

 the stain of No. 490 (113 mm.) does not permit of good histo- 

 logical study, yet it may be readily seen that the pontine nuclei 

 are still receiving great numbers of new elements from the rhom- 

 bic lip. At 143 mm. (No. 508) the production of neuroblasts 

 destined for the basilar parts of the hind brain has diminished very 

 appreciably. The mitoses around the ventricular margin are 

 fewer and the ependymal lining has begun to be separated quite 

 sharply from the underlying nervous tissue. Many of the cells 

 in the path of migration have larger and clearer protoplasmic 

 bodies, apparently unwilling to complete their journey to the 

 pontine region. Others appear as all of those in earlier stages 

 with elongated almost naked nucleus pushing on toward the pons 

 before assuming the ganglionic form. Of the cells which have 

 long since gained their permanent position in the pons, many can 

 now be recognized as ganglion cells. The nucleus is very much 

 larger, although rarely showing a distinct nucleolus — the cyto- 

 plasm, paler than the framework in which it is embedded, is also 

 increased in amount. The greater number of cells have grown 

 slightly — possessing a smaller, more densely staining nucleus 

 surrounded by a clear protoplasmic envelope. 



During the interval between 143 mm. (No. 508) and 188 mm. 

 (No. 509) the formation of neuroblasts ceases entirely and the 



