DEVELOPMENT OF THE NUCLEI PONTUS IN MAN 51 



No. 22) all of the later stages invariably show nuclear material 

 in the position which we know will be occupied by arcuate nuclei. 

 There appears but one rational explanation which will harmonize 

 all of these apparently jarring facts which we have determined. 

 In human embryos at the beginning of the second month there is 

 an intramedullary migration of cells from the rhomboid lip to 

 make up the olive, toward the end of the month the path of migra- 

 tion becomes more and more superficial until many of the cells 

 actually cross the raphe before plunging into the medulla. In 

 the lower mammals the comparatively simple olivary complex 

 soon acquires its allotment of cells and when production of oli- 

 vary neuroblasts ceases in the roof attachment, those on the 

 surface soon find their way into the interior. In man, on the 

 contrary, before the olive has received all of its cells and while the 

 migration from the rhombic lip is still proceeding actively, neuro- 

 blasts which cannot be differentiated from those destined for the 

 olive, begin to wander over the surface among the vagus roots. 

 These elements stop on the ventral surface near the raphe and 

 constitute the anlage of the arcuate nucleus. Stated differently, 

 we are probably not dealing with arcuate formation in human 

 embryos of 20 mm. where a cell lamina lies on the surface of the 

 medulla in the place where we know the arcuate nucleus ought to 

 be. 



Just when the arcuate neuroblasts begin to descend from the 

 rhombic lip can only be conjectured; this uncertainty has led me 

 to call it ' olivo-arcuate migration.' Probably at 30 mm., as 

 exemplified by No. 86, most of the thick superficial sheet of cells 

 in the arcuate region represents a migration of olivary elements 

 (fig. 10). Here the deeply staining nuclei form a continuous 

 lamina over the ventral surface, the caudo-cephalic extent of 

 which corresponds to the spinal one-half of the olive. Even older 

 embryos present this pons-like structure as figs. 4, 5, and 6 illus- 

 trate. First in a fetus of 80 mm. (No. 172) does one meet with 

 any large number of superficial neuroblasts under the cerebral 

 one-half of the olive. Here almost the entire surface of the 

 medulla is the seat of cellular migration. From the cervical 

 flexure almost to the pontine nuclei the pyramidal tract is covered 



THE AMERICAN JOURNAL OF ANATOMY, VOL. 13, NO. 1 



