378 JOHN LEWIS BREMER 



The fate of these recurrent branches is undoubtedly degen- 

 eration and complete disappearance, as no record has ever been 

 made, to my knowledge, of their presence after the 31-mm. 

 stage. The course of this degeneration can be followed in my 

 specimens with a fair degree of certainty. At first the nerve 

 fibers of the branch, especially at their tips, become wavy and 

 shghtly separated from one another, thus causing the terminal 

 expansion mentioned above. Then follows a reduction in the 

 number of fibers in the proximal portion of the branch, near the 

 nerve roots. All gradations of this peculiar feature can be seen 

 in different embryos, from a scarcely noticeable change of size 

 in the two ends' of the branch to a complete severance of the 

 distal end from its origin, so that the nerve fibers, still readily 

 recognizable, lie in the mesenchyma entirely separated from the 

 medulla and from the main abducens nerve. Whether or not 

 this is preceded by the degeneration of the nerve cells in the 

 abducens nucleus, I have no means of judging, as the material 

 available is not stained to show the details of nerve-cell degen- 

 eration. No obvious differences in the cells of the nucleus are 

 noticeable in any of the specimens. Similar free fibers have 

 been noticed and figured before by me and by Thyng^ in relation 

 to the degenerating anterior hypoglossal roots, but the result 

 was not so striking in that case, as the nerve bundles were distally 

 connected, at least physically, with a live nerve; the degenerating 

 branches of the abducens nerve, with no connection at either 

 end, seem much more surprising. The final dissolution or 

 absorption of the disconnected nerve fibers is apparently accom- 

 pUshed by the action of phagocytes. The process, as shown in 

 figure 4, is indicated by the presence within the nerve bundle 

 of phagocytic cells in various stages of engorgement, as evi- 

 denced by vacuoles in the cell protoplasm; they are apparently 

 recruited from the surrounding mesenchyma, and pass out into 

 it again after engorgement, or perhaps they are cells of the perineu- 

 rium in a new role. The cell reaction is slight, as only a few 

 small phagocytes are ever found, showing that the process is 

 slow and gradual; but these few cells are easily recognized, as 



* See figure 1, x, and also Bremer, 1908, figures 1 and 2, x. Thyng, 1914, plate 3. 



