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reason for this is the great increase in diameter which is known to 

 take place in nerve fibers in Petromyzon, both centrally and peripherally. 

 The Miillerian fibers themselves grow much thicker at some distance 

 from the cell and many of the arcuate fibers arising from the "tuber- 

 culum acusticum", including those mentioned in the next paragraphs, 

 increase in diameter several times before they have crossed the median 

 raphe. I have shown (1908) that the motor fibers increase in some 

 cases from about one micron to twenty-five microns in thickness during 

 their peripheral course. Within the spinal cord also I have seen 

 indications that an occasional motor root fiber increases to the thick- 

 ness of a medium Miillerian fiber before making its exit from the 

 cord. In view of these facts I do not think it is safe to rely on the 

 study of transverse sections to determine the fate of Miillerian fibers. 



Ahlborn described a bundle of Miillerian fibers arising from the 

 spindle cells in the acusticum and decussating in the floor of the 

 oblongata. I denied that these were Miillerian fibers, that any true 

 Müllerian fibers arose from the acusticum, and that any Miillerian 

 fibers decussate in the medulla oblongata. Schilling states that at 

 the level of the N. acusticus there are only six or seven Miillerian 

 fibers on each side, and that only a part of these have decussated 

 (p. 443). Teetjakoff states that the Miillerian fibers all arise from 

 the ventral (motor) columns (p. 656) and describes a large bundle 

 which arise in the region of the N. acusticus and form a decussation 

 (p. 692). This certainly indicates a difference between the species 

 studied. I have reviewed my preparations of Lampetra and there is 

 no decussation in the medulla oblongata of giant fibers or of fibers 

 arising from giant cells. Teetjakoff gives no figure of the decussation 

 of these fibers but his description implies that there are a larger 

 number of Miillerian fibers than are found in either of the species 

 studied by Schilling and myself. In Lampetra there are on each side 

 nine giant fibers beneath the ventricle at the level of the N. acusticus. 



Teetjakoff agrees with the writer that the thick fibers described 

 by Ahlboen as arising in the spindle cells of the acusticum and 

 decussating in the floor of the medulla oblongata do not form part 

 of the system of Miillerian fibers. Teetjakoff confirms my description 

 of the enlargements of the acustic nerve fibers in contact with these 

 cells. I described the axones of the spindle cells as coarse fibers 

 which formed two decussations, one behind the level of the N. acusticus 

 and one near the N. oculomotorius. I thought the fibers were disposed 

 in essentially the same way as the internal arcuate fibers. 



Kappees (1906) and Schilling (1907) regard the tract that 



