MORPHOLOGY OF EYE MUSCLE NERVES 11 



HISTOGENESIS OF SPINAL SOMATIC MOTOR NERVES 



1. General description 



Somatic motor nerves make their first appearance in embryos 

 of Squalus acanthias of about 4.5 mm. and approximately 30 

 somites. With their advent begins the rhythmical bending of 

 the embryo. The first pair of somatic motor nerves to appear 

 are those of VanWijhe's seventh somites, the first somites differ- 

 entiated, and those which form the first permanent myotomes. 



Previous to the appearance of this first pair of somatic motor 

 nerve anlagen, no protoplasmic connections or plasmodesms be- 

 tween tube and somite are discernible. In the intercellular 

 space between neural tube and somite there is present a plas- 

 moid substance or a liquid with a minimal amount of coagulable 

 material, which, when treated by the usual reagents, assumes 

 a vacuolated appearance. By the use of such fixing fluids as 

 neutral 10 per cent formalin, followed by intense stains such as 

 acid fuchsin it is possible to demonstrate this coagulable sub- 

 stance with considerable clearness. By the- use of other fixing 

 fluids — the plasmolyzing action of which is less intense than that 

 of formalin — and the use of less powerful stains, the plasmoid 

 material is almost invisible and has usually been ignored in 

 embryological studies. Whatever may be the chemical nature 

 of the granules of this plasmoid substance, it does not have the 

 staining properties of protoplasm and its presence may not there- 

 fore be taken as evidence of a primary protoplasmic connection 

 between tube and somite, although it is indisputably the medium 

 through which the growing nerve anlagen find their way to the 

 adjacent myotomes. 



While the possibility that this plasmoid material may con- 

 tribute to the growth of the nerve paths may not be denied, yet 

 even so the contribution must be very slight or quite negligible, 

 since the amount of coagulable material in it is very small. To 

 represent it at all, as in figure 1, greatly exaggerates its distinct- 

 ness and amount as seen in sections. There is nothing in its 

 vacuolated structure that would suggest 'paths' or 'plas- 

 modesms' suggested by the Hensen hypothesis. Moreover, the 



