622 "Joiinial of Comparative Neurology and Psychology. 



abducens, are quite distinct from the subdivided anterior condy- 

 loid foramen, and may mark the fusion of more anterior vertebras 

 in the occipital bone. Such a foramen, though existing only for 

 a short time, might cause a weak spot in the forming bone near to 

 the median line, which might serve as a path for a chordoma, a 

 tumor of the notochord which arises (as shown by Williams^) not 

 infrequently in this region. 



In one embryo (fig. 3) a root anterior to the hypoglossal roots 

 runs to the jugular foramen, and ends just posterior to the glosso- 

 pharyngeal nerve; and in other youngerembryos the more anterior 

 of these aberrant roots run as though to pass behind the glosso- 

 pharyngeal nerve, instead of behind the vagus (fig. i). 



Another set of aberrant roots was found in these embryos. All 

 those mentioned heretofore, after emerging from the medulla on 

 either side of the median line, run ventrally, either forward to join 

 the abducens, or caudally to join the hypoglossal nerve, or at least 

 in the general direction of the hypoglossal roots. The fibers of 

 this other set, emerging also from the ventral part of the medulla 

 between the abducent and hypoglossal roots, take a lateral course, 

 at right angles to the hypoglossal roots, or even turn dorsally. 

 Such fibers are not uncommon, to judge by the embryos in the 

 Harvard Embryological Collection. They may arise separately, 

 or as branches of the roots of usual nerves; if they are long enough 

 we can follow them to the mesenchyma at the side and back of 

 the head. They may be looked upon as of two groups: those 

 more caudal, arising with, or just anterior to the roots of the 

 hypoglossal nerve, and those still more anterior, arising with or 

 just posterior to the abducens. The fibers of the first group run 

 to the back of the neck by passing behind the vagus and accessory 

 nerves, w^hile those of the second group pass in front of the vagus, 

 behind the glossopharyngeal nerve. In one embryo (fig. 5) as 

 was the case with the ventrally running roots, fibers from the same 

 root diverge, one branch running in front of, one behind, the con- 

 joined vagus and accessory. In other cases the lateral fiber is 

 made by the junction of several roots, even including a branch 

 of the abducens. 



One of these fiber bundles with a dorsal course may be seen in 

 fig. 4, a sagittal view of a human embryo of 11. 5 mm. which shows 



'Williams, L. W.: Paper in press. 



