374 JOHN LEWIS BREMER 



nated by the loss of the ventral roots between the two definitive 

 nerves, so that the nerve origins become separated by a widening 

 gap. None of these transient rootlets or connections grows 

 toward the vagus or glossopharyngeal nerves, and there is no 

 evidence that any fibers have an independent growth caudally 

 from the permanent abducens roots. Belogolowy examined also 

 pig embryos for comparison of the head nerves with those of 

 chicks, and figures in an embryo of 3 mm. frontoparietal measure- 

 ment (about 8 to 9 mm. greatest length) short rootlets continuing 

 the ventral series between the abducens and hypoglossal, but 

 without joining either of these nerves. These I have also found 

 in the majority of the 125 class specimens of 10- to 12-mm. 

 pig embryos cut in transverse section (though in this plane they 

 are not conspicuous) and in some of the younger specimens in 

 the collection. None of these is of more than a few fibers, and, 

 as Belogolowy noticed, none remains for longer than the growth 

 period of 5 or 6 mm. 



In sheep and rabbit I find even these rootlets much less fre- 

 quently, so that a recurrent branch of the abducens must be 

 very rare in these embryos. In many of the rabbit embryos of 

 thirteen or fourteen days the anterior XII roots run between the 

 vagus and the glossopharyngeal nerves, but these seem to dis- 

 appear whoUy after this time. They have no connection with 

 the abducens, all of whose fibers point immediately forward. 



In Lacerta muralis, between 5 and 7 mm. in length, and in 

 Chrysemys marginata, from 7 to 9 mm., the abducens is provided 

 with a few posterior rootlets, joined in loops, behind the perma- 

 nent roots. In this respect these two species of reptile resemble 

 the birds (chick), but the rootlets do not extend as far caudally 

 in the reptiles, and there is never, apparently, a continuous 

 connection between the abducens and the hypoglossal. I have 

 found no remains of these rootlets in older embryos. 



From this brief resume of the findings in Acanthias, Heptan- 

 chus, chick, reptile, pig, sheep, and rabbit, it is clear that transient 

 nerve roots arising from the ventral surface of the hind brain 

 between the permanent abducens and hypoglossal roots are to 

 be expected in a certain, usually small, percentage of these 



