110 EMBEYOLOGY OF THE LOWER VERTEBRATES 11. 



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

 present as a distinctly tibri Hated 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. 



l-'i.u- 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 fix in- 

 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 (?i) 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 



FIG. 61. Part of transverse section of Lepidosiren take the Various stages in 



(stage 34), showing a portion of nerve-trunk. ^eir proper chronological 



/////, myotome; N, notochoi <l ; , IMTV. --trunk ;.n.S, nucleus seQUCUCC. 



sl..-ath; ,Si, primary sli.-ath of nutochonl ; XI, ,-, x rfi nprvpfvimk ic 



lateral teaaeh of Yagoa. W ine nelve ' 



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 

 iibrillated. 



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

 chvmatoiis protoplasm collect round it and gradually spread out 

 over its length to form the protoplasmic sheath. 



In later than those li^mvd the sheath protoplasm insinu- 



LBclf in amongst the nerve-lihrils of the trunk, dividing them 



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



into the \arious muscles of the adult each piece retains its primiti\e 



nerve-strand, drawing if with it. as it becomes pushed about b\ the 



pp., different lal growth, as its .\\n special nerve. 



It should be mentioned that the most important point in the 



