128 EARLY DEVELOPMENT OF MOTOR NERVE TRUNKS AND MYOTOMES. 
Fig. 3. Part of transverse section at stage 27. (69C. 1064.) Showing whole length of motor nerve 
trunk. my. myotome ; 2. notochord ; .¢, motor nerve trunk; p.s. protoplasmic sheath now in form of a 
mass of richly yolked mesenchymatous protoplasm aggregated round the nerve trunk near its outer end, 
The greater part of the nerve trunk is still naked. At its outer end the nerve trunk passes out into conically 
arranged strands of protoplasm forming the inner ends of the muscle cells, 
Fig. 4. Part of transverse section at stage 25. (79 A. 933.) -my. myotome ; n. notochord ; 7.¢. motor 
nerve trunk already faintly fibrillated passing between spinal cord (s.c.) and myotome. ‘The mesenchyme 
(me.) has not yet begun to concentrate round the nerve trunk. 
Fig. 5, Part of transverse section at stage 24. (73G. 986.) my. myotome; m. notochord ; n.¢. motor 
nerve trunk, at this time composed of granular protoplasm, and naked. 
Fig. 6. Similar section to last, but taken from rather less advanced specimen. (Stage 24; 73 F. bes. 
my. myotome; n.f. motor nerve trunk—a naked protoplasmic bridge connecting myotome and spinal 
cord (s.c.). 
Puate II. 
[All figures are camera drawings of single sections. Figs. 7, 8 and 9 represent the same sections as are 
photographed in figs. 2, 3 and 4, with the additional detail visible under Zeiss’ 3 mm. apochromatic 
homogeneous immersion objective. ] 
Fig. 7 (=fig. 2). The motor nerve trunk (n.t,) is seen within its thick protoplasmic sheath (>p. s.). 
Fig. 8 (=fig. 3). The motor trunk is now naked except for the large mass of yolk-laden 
mesenchymatous protoplasm (.s.) which has concentrated round it towards its outer end. The continuity of 
nerve trunk with protoplasm of muscle cell is seen. 
Puate III. 
Fig. 9 (=fig. 4). The nerve trunk is seen to be already fibrillar in structure. Mesenchyme (me.) 
has penetrated in between myotome and spinal cord, but has not yet begun to concentrate round the nerve 
trunk to form its sheath. 
Fig. 10. Stage 24. The nerve has been torn away from the inner surface of the myotome so that its 
expanded outer end is seen. of 
Fig. 11. Part of transverse section, stage 30. (93 B. 2844.) The two walls of the myotome are seen— 
the outer one-layered, the inner composed of a layer of myoblasts in which are seen the first contractile 
fibrils (c.f). 
Fig. 12. Part of transverse section, stage 31. (103 D. 2463.) The outer wall of the ayotsmen is beginning 
to show more than one layer of nuclei. . In the myoblasts of the inner wall the contractile fibrils (cf.) have 
greatly increased in number, most of the protoplasm of the inner half of the cell being converted into fibrils. _ 
Prats IV. 
Fig. 12a, Part of longitudinal horizontal section at stage 31 (103 C. 822), showing a single myotome. 
zw. Inner wall cell; c.f. contractile fibrils; g. glycogen-containing outer part of cell; mw. nuclei of inner 
wall cell; y. yolk granules; 0.w, outer wall of myotome, two of the nuclei undergoing mitosis; me. 
mesenchyme nuclei of septum between myotomes. 
Fig. 13, Part of transverse section at stage 31+. (106 C. 1573.) The outer wall of the myotome is 
now several layers thick, and the cells of this wall have also developed contractile fibrils (v.f.). 
Fig. 13a. Longitudinal horizontal section through a myotome of stage 31+. (106 A. 1811.) The 
nuclei of the inner wall myoblast are seen to have considerably increased in numbers. The contractile fibres 
of the outer wall (c,f.) are visible. i 
Fig. 14. A nucleus of one of the inner wall myoblasts during mitosis, showing the cell-like demarcation 
of the protoplasm immediately surrounding it. 
t 
