EARLY DEVELOPMENT OF MOTOR NERVE TRUNKS AND MYOTOMES 127 
so gaps are bridged over; and (2) as soon as it has become continuous, nerve impulses 
beginning to play backwards and forwards in its substance cause again a differentiation 
into fibrils. As part of the impulse tracks persist as the stumps of the fibrils, the 
regenerate parts of the fibrils will naturally develop in exact continuity with these. 
I have no intention of entering into a general discussion either as to the data in 
connection with the development of nerves in other Vertebrates, or as to the general 
conclusions which have been based on them. My purpose now is merely to emphasize 
the observations of the phenomena as they occur in Lepidosiren. I may, however, be 
allowed to point out that such observations as those of Berne upon the Chick,* although 
apparently supporting the cell chain view, are in no way irreconcilable with the 
observations here chronicled. The soft protoplasmic bridges which form the first 
distinguishable rudiment of the motor nerve are difficult to observe in Lepzdosiren, 
whose histological features are upon a relatively large and coarse scale; and how much 
more delicate and difficult to observe are the corresponding structures likely to be in 
the chick! It may well be that further research will demonstrate the existence of a 
delicate protoplasmic bridge about which are clustered the ‘nerve cells” (in APATHY’s 
sense) observed by upholders of the cell chain theory. 
DESCRIPTION OF PLATES. 
GENERAL List or ABBREVIATIONS. 
ef. Contractile fibrils. nu. Nucleus. 
d.r. Dorsal root. n.c. Neural canal. 
g. Glycogen-containing outer portion of myoblast. n.s. Nuclei of protoplasmic sheath. 
z.w. Inner wall of myotome. n.t. Nerve trunk. 
l.vag. Lateral branch of vagus. o.w. Outer wall of myotome. 
me. Mesenchyme. p.s. Protoplasmic sheath of nerve trunk, 
m.n.r. Motor nerve root. s.c. Spinal cord. 
my. Myotome. v.r. Ventral root. 
nm. Notochord. y. Yolk granules, 
PuateE I. 
[The figures on this plate are all from untouched negatives. }] 
Fig. 1. Portion of spinal nerve of stage 34. (108°2835.) Jl.vag., lateralis vagi; my. myotome ; 
n. notochord ; n.¢. nerve trunk; 7.s. nuclei of protoplasmic sheath; d.r. dorsal root; v.r. ventral root. 
Fig. 2. Portion of spinal nerve trunk of stage 29.* (93 C. 1435.) my. myotome; x. notochord ; 
nt, motor nerve trunk ; p.s. protoplasmic sheath containing yolk granules (black), and large nuclei rich in 
chromatin. 
* Op. cit., p. 238. 
+ Much of the minute detail has unfortunately disappeared in the mechanical printing of these figures. I shall be 
glad therefore to show any specialists who are interested sun prints from the same negatives, in which the full detail 
is brought out. ; 
TRANS. ROY. SOC. EDIN., VOL. XLI. PART I. (NO. 7) 21 
