52 CHARLES R. ESSICK 



by a superficial sheet of cells many of which are pushing their 

 way into the medulla near the raphe. The wandering of cells 

 to the region of the cerebral half of the olive fills up, in this em- 

 bryo, the gap between the pons and the band (olivo arcuate 

 migration, (fig. 2) which was first completed in embryos of 20 mm. 

 The addition of new elements is even more marked in the 96 

 mm. fetus (No. 484) where great numbers of moving cells are 

 directed toward the ventral portion of the medulla immediately 

 behind the pons. Here the cells are leaving not only the rhombic 

 lip to pursue a course similar to the earliest olivo-arcuate migra- 

 tion but also from the ventral edge of the thick column of migrat- 

 ing pontine nuclei. All along the corpus ponto-bulbare of this 

 fetus neuroblasts can be seen to leave its ventral edge and migrate 

 directly toward the ventral median fissure. In the adult it is 

 well known that the arcuate nuclei fuse across the midline as 

 one nears the pons, although at the caudal end of the olive they 

 present two discrete swellings which lie some distance from the 

 midline. Two mechanical factors are concerned in breaking up 

 this uninterrupted sheet of nuclear material which is so striking 

 in the younger fetus (fig. 10). These are the formation of the 

 external arcuate fibers and the growth of the pyramidal tract. 

 Already in this fetus a considerable number of arcuate axones are 

 crossing in the raphe, the main mass of nuclei, however, still lie 

 on either side of the midline (fig. 11). It remains for the con- 

 stant interstitial addition of pyramidal axones to bring about the 

 further separation of the arcuate nuclei. It is apparent that the 

 cortical projection system must occupy a very inconspicuous 

 part of the cross sectional area of the medulla of this embryo, 

 when one considers the superficial position of the olivary complex 

 and their proximity to the ventral medial fissure. Compare with 

 this the fig. 12 which is a camera lucida tracing of No. 508 (143 

 mm). The level of this section was made to correspond with that 

 of No. 484 by choosing both about one-tenth of the distance from 

 the caudal to the cephalic pole of the olive. The extensive addi- 

 tion to the pyramidal tract has pushed the olives apart as well as 

 the arcuates. The latter, remaining superficial to the rapidly 

 growing nerve system, have been drawn away from one another. 



