EARLY DEVELOPMENT OF MOTOR NERVE TRUNKS AND MYOTOMES. 121 
Stage 24.—Transverse sections about the middle of the body at this stage (figs. 5 
__ and 6) show the myotome still in contact with the wall of the neural canal, mesenchyme 
either not having intruded itself between the two at all, or only having done so to a 
very slight extent as yet except in the anterior part of the body. In embryos which 
have been excised while alive in normal salt solution, spread out in one plane like that 
figured to illustrate this stage in my account of the external features of development,* 
and fixed in this position, the myotomes are pulled slightly away from the neural tube. 
It is seen in transverse sections of such embryos that the rudiments of the motor nerve 
trunks already exist as soft thin bridges (n.¢.) metamerically arranged and connecting 
spinal cord rudiment and myotome as shown in figs. 5 and 6. 
The nerve rudiments at this early stage are formed of granular protoplasm either 
without yolk or containing only very minute granules, without obvious fibrillar 
structure when stained by the ordinary methods. The rudiment is quite naked, the 
richly yolked mesenchymatous sheath of later stages being conspicuous by its absence. 
Nor are there any nuclei in contact with the nerve rudiment. There can happily be 
no doubt about this in Lepidosiren, where the nerve rudiment in this early condition is 
of relatively small size compared with the dimensions of a single nucleus ! 
The nerve rudiment—composed as it is of soft protoplasm—is at first extremely 
fragile, gradually becoming tougher as development proceeds. Consequently in a 
straightened out embryo of this period we find the more posterior nerve rudiments—.e. 
those in an earlier stage of development—show a greater and greater tendency to be torn 
away from the myotome in prepared sections. A nerve rudiment which has become 
torn away from the myotome is shown in fig. 10, which brings out a point more difficult 
to observe in the uninjured condition, that the protoplasmic mass forming the nerve 
rudiment spreads out over the inner face of the myotome. How far this expansion 
extends, whether—as is probable—it really covers the whole face of the myotome, is a 
point almost impossible to decide definitely by actual observation. Similarly [ am 
deterred by the unreliability of observations made on a spinal cord so laden with yolk 
in its early stages from saying anything as to the connections of nerve rudiments with 
neuroblasts or other cellular elements in the substance of the spinal cord. 
At this stage the spinal cord is without any obvious mantle of fibres. 
The motor nerve trunk has thus been traced back to a period in which it is repre- 
sented by a bridge of soft granular protoplasm connecting spinal cord and myotome at 
a stage when these structures are in close apposition. As the myotome becomes pushed 
outwards by the development of mesenchyme, it remains connected with the spinal cord 
by the ever-lengthening strand of nerve. As the nerve develops it soon loses its simple 
granular protoplasmic character and assumes a fibrillated appearance. Richly yolked 
masses of mesenchymatous protoplasm become aggregated round the nerve, which till 
now has been quite naked. At first this protoplasm forms an irregular mass towards 
the outer end of the nerve trunk, but it soon spreads along it and forms a definite sheath. 
* Phil. Trans. B., vol. cxcii., pl. 10, fig. 24. 
