50 CHAELES E. ESSICK 



toward the portion of the medulla which is under the partially 

 formed olivary complex and recalls the undifferentiated wandering 

 cells seen in connection with the nuclei pontis. A great many 

 leave the surface at various points and plunge into the depth to 

 join the neuroblasts already massed up in the olivary nuclei. A 

 broad sheet, however, remains superficially and can be traced 

 from the roof attachment ventrally. In some brains (Nos. 368 

 and 453) this sheet has moved among the vagus rootlets and 

 advanced almost to the emerging hypoglossal roots. In another 

 (No. 22) the migrating cells cover the entire ventral surface of 

 the medulla just in front of the cervical flexure having met, across 

 the raphe, those moving down from the opposite side. In other 

 words, there exists at this period a band of superficial undifferen- 

 tiated cells uniting the roof attachment on both sides which is 

 not unlike the early pontine bridge in fetus of 30 mm. The 

 former begins just in front of the cervical flexure and subtends 

 one-half to two-thirds of the olivary complex (fig. 2), but unlike 

 the latter many cells leave it everywhere and make their way into 

 the substance of the medulla to form gray matter in the interior. 



It is striking (1) that the neuroblasts of the developing arcuate 

 nuclei imitating the pontine formation, pay no attention to the 

 raphe but cross it in an uninterrupted sheet ; (2) that they appear 

 before the anlage of the nuclei pontis, and if one turns to mamma- 

 lian embryos (I have studied pig and rabbit) which are slightly 

 larger than 20 mm. in crown-rump measurement, (3) that a well- 

 developed arcuate formation exists just as in the human material. 



Turning to the adult brain we find each arcuate nucleus a dis- 

 crete mass which is separated from its counterpart by the raphe. 

 Moreover the arcuate nucleus is peculiar to man so that from a 

 phylogenetic standpoint we should expect to find it developing 

 later than the pontine nuclei inasmuch as it is last to be acquired. 

 Furthermore, very soon after its formation in pigs and rabbits one 

 looks for it in vain. At 51 mm. only comparatively few cells can 

 be found, while the superficial layer of migrating cells has dis- 

 appeared completely from the subolivary region of a fetal pig 

 of 60 mm. In man, on the other hand, when once there is a col- 

 lection of cells over the medulla in the subolivary region (as in 



