Explanation of Plate XIII, figs. 12-28:

'Fig. 12. To show still further absorption of myeline, the increase in nuclei and the way they are grouped round the balls of myeline. The collapsed tube between the nuclei. A bit of endoneural sheath lying at the side of the fibre. Exp. VII. 14 days. R. S. Gold and haernatoxylin.

Fig. 13. To show the nearly complete absorption of myeline and the nuclei lying free in the tubes. Exp. VII. 14 days. R. S. Gold and haematoxylin.

Fig. 14. To show the irregularity of absorption. A portion of the tube free from myeline next to a portion still showing large segments. Exp. VII. 14 days. L. S. Gold and haematoxylin.

Fig. 15. The same. Exp. VII. 14 days. R. S. Gold and haematoxylin.

Fig. 16. To show the beginning of the formation of an embryonic fibre, the growth of protoplasm at the nucleus. Exp. XIV. 3 weeks. R. S. 3 in. below wound. Gold and haematoxylin.

Fig. 17. The same. Later stage. Exp. XIV. 3 weeks. L. S. 3 in. below wound. Gold and haematoxylin.

Figs. 18, 19, 20. To show stages in the formation of the embryonic fibres, 18 from Exp. IX. R. S. (3 weeks), 19 from Exp. IX. R. S. (3 weeks) and 20 from Exp. VIII. R. S. (3 weeks). Gold and haematoxylin.

Fig. 21. To show the flattened shape of the embryonic fibres often seen in the preserved specimens. Exp. VIII. 3 weeks. L. S. Below wound 3 in. Gold and haematoxylin.

Fig. 22. To show the large balls of myeline in the plasma cells lying between the fibres. Exp. XVII. R. S. 4 weeks. 4 in. below wound. Osmic acid and haematoxylin.

Figs. 23, 24. To show fully formed embryonic fibres and appearance of new sheath. Exp. XXVIII. 3 weeks. Br. to flex.-profundus. Osmic acid and haematoxylin.

Figs. 25, 26, 27, 28. From the peripheral end of human ulnar 6 1/2 months after section, union with central end not made. To show formation of "embryonic fibres." Gold and haematoxylin.' (402-403)

Figs. 12-15 in text:

'From the 7th day to the 14th day the process of absorption of the balls of myeline with their contained fragments of axis cylinder goes on actively, yet quite irregularly. Fig. 10 from a nerve after 7 days shows very well the breaking-up of the myeline and the formation in between, especially at the nuclei, of an apparently liquid substance in which are contained numerous fragments of the old myeline. As the absorption progresses and the fragments of myeline become smaller and less numerous the direct participation of the nuclei in the process becomes more evident. The nuclei are much more numerous and are found clustered in and about the remaining balls of myeline as shown in Figs. 11 and 12, representing a degeneration of 9 and 14 days respectively. One often sees bits of the myeline partially imbedded in a nucleus, and this appearance is found from this time on well into the later stages of regeneration, as long, in fact, as any of the myeline remiains unabsorbed. After 14 days absorption has gone so far that long stretches may be found, as shown in Figs. 12 and 13, in which only small fragments of myeline are present. At such places the fibre consists of a homogeneous, apparently liquid substance lying in the old sheath, and of many nuclei, often in pairs or groups, the latter giving indication of an active proliferation. Yet at this time, 14 days, and even later, one sees many fibres in which the absorption has lagged behind the condition of what may be considered a typical fibre of this period. In one and the same fibre places will be found in which absorption has made rapid progress in spots, all the myeline having disappeared, while in other spots the large cylindrical segments have suffered scarcely any change. Examples of this are pictured in Figs. 14 and 15. However irregularly the process may go on the final outcome is the complete absorption of the remnanits of the old myeline and axis; though as we have said before, balls of the myeline may be found in certain fibres long after this period, even at the time when fully formed new fibres have beeu produced. As the absorption proceeds the old sheath collapses more or less. It seems at first to contain a liquid material with some debris of the old myeline, but this too finally disappears and the beginning of the actual process of regeneration is inaugurated by the formation of new protoplasmic material around the numerous nuclei contained in the fibres.' (373-374)

Figs. 16-24 in text:

'Regeneration in the Peripheral End:- The increase of protoplasm round the nuclei is shown in its different stages in Figs. 16, 17, 18. At first, where the nuclei lie singly, the new protoplasmic formation gives the appearance of spindle-shaped cells or fibres such as have often been described as taking an active part in the formation of the new fibres, - Fig. 16. Later the increased formation of new material results in filling the old sheath with a continuous band or fibre of protoplasm in which the nuclei are imbedded, as shown in Figs. 19, 20, 21, 22, 23, 24. The fibre as now formed represents the completion of the first stage in regeneration.' (374)

Fig. 21 in text:

'The embryonic fibres, it should have beeni stated, often appear in the hardened specimens as flattened ribbons, instead of cylindrical threads, as shown in Fig. 21.' (375)

Figs. 23-24 in text:

'After the formation of the complete embryonic fibre a new sheath is made by a differentiation of the peripheral layer of the protoplasmic thread. The appearances which have led us to this view are shown in Figs. 23 and 24, in which the newly-formed fibre with its sheath lies within the old sheath. The old sheath is displaced outward and one may suppose that it eventually forms a part of the endoneural connective tissue which is at this time abtundant.' (375)

Fig. 23 in text:

'there is a strong probability that many of the old fibres do not succeed in regenerating, but go to ground completely. One often meets specimens such as are represented in Fig. 23, in which large balls of myeline are found on the sides of new fibres, engulfed in the cytoplasm of the wandering cells.' (375-376)

Fig. 24 in text:

'specimens like that represented in Fig. 24 may explain what Schiff and others have mistaken for a persistent axis cylinder. Certainly our modern methods of staining prove that after the first week or ten days there are no distinguishable fragments of the old axes remaining in the fibres of the peripheral end of the nerve.' (377)

Figs. 25-28 in text:

'Our longest experiments upon a nerve cut but not sutured, and in which reunion was prevented by removing a piece of suitable length, were experiment XXVI., upon a rabbit examined 70 days after the operation, and XXII., central ulnar of dog, 75 days. In addition, however, we have had the opportunity of examining a portion of the peripheral end of a human ulnar nerve which had been cut accidentally six and a-half months previously. We owe the opportunity to the kindness of Dr T. A. Mc Graw, who in an operation for secondary suiture removed small portions of the central and peripheral ends and sent them to us in Mueller's solution. Figs. 25, 26, 27, 28 are from drawings of specirmens prepared from the peripheral end of this nerve after treatment with the potash gold stain of Freud and Böhmer's haematoxylin. They show that regeneration had advanced to the stage of the formation of embryonxic fibres, but that there were no signs of an axis cylinder or myeline sheath. Similar preparations were obtained from the animals upon which section without suture was made. Cross sections of the peripheral end of the human nerve are shown in Fig. 29, stained with gold and haematoxylin and in Fig. 30, stained in haematoxylin alone. The drawing in Fig. 29 was made from a portion of the nerve in which some of the nuclei show a central nucleolus in cross section, simulating the appearance of. new nerve fibres with a central axis. This is better shown in Fig. 31, taken from the ulnar nerve of a dog after sutture but before regeneration.' (376-377)