134 EDWARD L. RICE 
(pr.b-pt.). No conspicuous variation is noted in the different 
stages studied except on one side in stage 6. Here the tunnel 
doubly penetrates the basal plate, and for a short distance the 
nerve (fig. 20, left side, n.VJ.) lies in an open groove on the ven- 
tral side of the cartilage. There is no evidence that this variation 
is due to the more advanced age of this embryo (the tunnel on 
the other side is not unusually extended toward the ventral side), 
nor that it has any special morphological significance; much 
more probable is the view that it is a mere accidental or indi- 
vidual variation parallel to the variations already described for 
the hypoglossus foramina. 
While there is seemingly unanimous agreement concerning the 
course of the abducens in the reptiles, one point in the interpre- 
tation of this course demands consideration. It is usual to 
interpret the passage of the nerve through the crista sellaris as 
its emergence from the primary cranial cavity; thus Gaupp 
(11) and Voit (09 b), both of whom contrast this exit in the 
reptiles with the exit in mammals through the fenestra sphen- 
oparietalis of the lateral wall. But examination of Gaupp’s 
figures of Lacerta or of figure 1 of this paper will show conclu- 
sively that, when it emerges from the tunnel in the crista sellaris, 
the nerve still lies dorsal to the trabecula (trab.) and median to 
the pila prootica (pi.pr-ot.), rudimentary in stage 5 of Eumeces; 
in other words, it is still within the primary cranial cavity, and 
finds its real exit through the rather indefinite connective tissue 
filling the fenestra metoptica of the lateral cranial wall (see de- 
scription of temporal region, p. 179). These points have been 
correctly emphasized by Shiino (’14). This interpretation brings 
the course of nerve VI in the reptile into harmony with that in 
the mammal except that the mammal shows no perforation of 
the crista sellaris. Even in this respect the bridging of the 
abducens by a bar of cartilage from the ‘processus clinoideus 
posterior’ of the basal plate to the cochlear prominence of the 
ear in the pig (Mead, ’09) is very suggestive of conditions in the 
reptile, although the fenestra thus enclosed is not strictly homol- 
ogous with the hypoglossus foramen of the reptile, as pointed 
out by Mead himself. The notch for the abducens on the lateral 
