48 CHARLES R. ESSICK 



cleus at first separated from the olivary body by a considerable 

 interval comes to lie in the same transverse section as the latter, 

 while the pontine nuclei cover up the cephalic two-thirds of this 

 nucleus. We have, then, in addition to the mere increase in size 

 of the pontine and olivary nuclei an actual alteration of their posi- 

 tions as a result of the straightening out of the neural tube. As 

 a consequence nuclear masses which were separated from one 

 another, are crowded together and the course of the sixth cranial 

 nerve altered. 



Having considered the origin of the main mass of nuclei pontis, 

 the possibility of cells from other sources must not be overlooked. 

 In the region of the pontine flexure near the raphe one can make 

 out at an early period collections of cells extending from the ven- 

 tricular floor to the pontine nuclei with which they are connected. 

 They occupy the position which is held by the nuclei reticularis 

 tegmenti pontis (Flechsig) in the adult. Long before any cells ap- 

 peared superficially on the pontine flexure the karyokinetic figures 

 had disappeared in the ependymal sheet near the raphe, so that it 

 is highly improbable that the nuclei pontis depends on this portion 

 of the neural tube for many of its elements. In addition these 

 cells of the nuclei reticularis tegmenti pontis are evident long 

 before the pontine nuclei appear and never have the characteristic 

 appearance of young wandering neuroblasts during pontine de- 

 velopment. In some of the older embryos a thin sheet (one to two 

 cells deep) are migrating from the wall of the lateral recess in front 

 of the dorsal cochlear nucleis but the layer is narrow and composed 

 of comparatively few cells. These cells join the pontine nuclei 

 behind the trigeminal nerve. It is hardly necessary to exclude 

 other sources if one considers seriously the great production of 

 new cells around the rhombic lip. This begins at 23 mm. and con- 

 tinues incessantly until the fetus has passed 143 mm. in crown- 

 rump measurement. Couple with this extensive period the short 

 time in which any mitotic division is complete and the great 

 numbers met with in every section and it will not take a great 

 stretch of imagination to account for all of the cells in the nuclei 

 pontis. 



