380 JOHN LEWIS BREMER 



usual course of the recurrent branch, made me reasonably certain 

 that this group of cells represented the last stage in the process 

 of dissolution of this nerve. 



Usually the recurrent branches either degenerate relatively 

 early, before the precartilage of the base of the skull is laid down, 

 or if they persist pass toward or through the jugular foramen 

 with the vagus and glossopharyngeal nerves. On the left side, 

 however, of one embryo of 25 mm. (H. E. C, no. 2042) the 

 long well-marked recurrent branch, larger than the main abdu- 

 cens nerve, passes through a separate foramen, walled off from 

 the jugular foramen by a substantial plate of precartilage, and 

 containing in addition to the recurrent branch of the abducens 

 a meningeal branch from the trunk of the vagus, and a small 

 vein, probably the inferior petrosal sinus. These two structures 

 commonly accompany long recurrent branches of this type, but 

 the partition of precartilage across the jugular foramen I have 

 seen in no other embryo. It probably marks off the anterior 

 compartment of the jugular foramen mentioned in some text- 

 books of anatomy as containing the inferior petrosal sinus, but 

 in other embryos of this age, and even considerably older, the 

 cartilaginous plate does not exist, nor is it present on the right 

 side of this embryo, where the recurrent branch is much shorter. 

 The possibility is strongly suggested that this early subdivision 

 of the jugular foramen is caused by the presence of the recurrent 

 branch, and a bony canal in this position might even be looked 

 for as an anomaly. 



In attempting to explain the fact that the recurrent branches 

 of the abducens nerve attain in man their greatest duration and 

 size and frequency, obviously phylogenetic considerations are 

 of no assistance. Nor can Ave turn with any hope of enhghten- 

 ment to the history of the head somites, for they are not recog- 

 nized generally in man, in spite of a paper by Zimmerman, who 

 reported finding them in an embryo of 3.5 mm. We are forced 

 to an examination of the nerves themselves at their earher stages, 

 and of any adjacent structures which might influence them, in 

 the search for a possible explanation. 



