110 EMBEYOLOGY OF THE LOWEE VEETEBEATES ch. 



n.~ 



ll J 



Xt. 



recede from the spinal cord, but yet each motor nerve is already 

 present as a distinctly -fibrillated trunk bridging across the narrow 

 gap between spinal cord and myotome. A few mesenchyme cells 

 have wandered into the gap but they have not yet begun to con- 

 centrate round the nerve-trunk. 



Fig. 60, A is taken from an embryo of stage 24 at a time when 

 myotome and spinal cord are still in close contact with one another. 

 In specimens which were extended in one plane under normal salt 

 solution while still alive and subjected to the action of the fixing 

 agent in that position, it is found that the myotome is frequently 

 pulled slightly away from the spinal cord (as in the specimen 



figured) and in such cases it 

 is found that the nerve- 

 trunk already exists in the 

 form of a bridge of soft 

 granular protoplasm (n) with- 

 out any trace of fibrillation, 

 connecting spinal cord and 

 myotome. That these bridges 

 are really the nerve -trunks 

 is indicated by their occur- 

 rence one to each myotome, 

 apart from the fact that a con- 

 tinuous series of stages have 

 been observed between them 

 and the fully developed 

 nerve-trunks. 



In summing up we may 

 take the various stages in 

 their proper chronological 

 sequence. 



(1) The nerve-trunk is 

 already present as a proto- 

 plasmic bridge at a period so early in development that spinal cord 

 and myotome are still in contact with one another. 



(2) As the embryo grows and the myotome recedes from the 

 spinal cord this protoplasmic bridge increases in length and becomes 

 fibrillated. 



(3) As the nerve-trunk lengthens amoeboid masses of mesen- 

 chymatous protoplasm collect round it and gradually spread out 

 over its length to form the protoplasmic sheath. 



In stages later than those figured the sheath protoplasm insinu- 

 ates itself in amongst the nerve-fibrils of the trunk, dividing them 

 up into bundles or nerve-fibres. As the myotome resolves itself 

 into the various muscles of the adult each piece retains its primitive 

 nerve-strand, drawing it with it, as it becomes pushed about by the 

 processes of differential growth, as its own special nerve. 



It should be mentioned that the most important point in the 



Fig. 61. — Part of transverse section of Lejndosiren. 

 (stage 34), showing a portion of nerve-trunk. 



my, myotome; N, notoehord; n, nerve-trunk ; n.S, nucleus 

 ot nerve sheath ; S 1 , primary sheath of notoehord ; XI, 

 lateral branch of vagus. 



