210 bulletin: museum of comparative zoology. 



h. Nerve Relations in the Trunk of S. acanthias. 



An examination of sections in the trunk region of embryos of S. acan- 

 thias leaves no doubt whatever that the chief proliferation of ganglionic 

 cells occurs in the regions of constriction between myelomeres, i. e. 

 opposite the somites, and that the ventral roots also arise opposite the 

 somites. Motor roots appear long before the sensor roots, as was first 

 stated by Sagemehl ('82). Dohrn has affirmed that they arise as early 

 as Balfour's stage H. I find them in embryos of S. acanthias in which 

 34 somites are differentiated, stage H. From the very first, i. e. at 

 this early stage, they are in relation with the ventral portion of the 

 neural tube at a point directly opposite the middle of the somite. That 

 the relation with the tube is opposite the middle of the somite is most 

 easily demonstrated in frontal sections (see Plate 6, Fig. 42, which 

 represents a frontal section of an embryo with 50 somites); but that 

 their relation is with the ventral wall of the tube, is most cleai'ly seen 

 in cross sections (Plate 6, Fig. 41, rx. v.). In frontal sections more 

 dorsally situated than those which show the ventral roots, the spinal 

 ganglia are likewise seen to lie opposite the middle of the somites^ 

 (Plate 6, Fig. 43). In later stages, however, the spinal ganglia lie 

 opposite the anterior portion of the somites, i. e. intersomitic in position, 

 as a result, probably, of the shifting of the somites. Since by this time 

 the constrictions between myelomeres have disappeared, it is qxiite im- 

 possible to state that doi-sal roots arise either from the constrictions or from 

 the dilatations of the myelomeres. 



McClnre ('90, p. 42) has said that in the forms studied by him " the 

 dorsal branches of the spinal nerves pass from the external surface of the 

 myelomeres to the space between two somites, which is opposite their 

 point of origin, and fuse with the epiblastic thickenings to form the 

 spinal ganglia." Such a statement, if true, is certainly of great impor- 

 tance in settling the question of the morphology of cranial nerves. For 

 it is now generally stated by morphologists that the chief distinction be- 

 tween spinal and cranial nerves consists in the fact that the ganglia of 

 cranial nerves receive cellular material during development from the 

 ectoderm of the lateral surface of the head, whereas the spinal ganglia 

 do not. So far as I know, McClure's statement. remains unconfirmed, 



1 Similar relations of dorsal ganglia and ventral roots have been shown by Mar- 

 shall ('78, Plate IIL Figs. 27 and 28) for birds ; by Hoffmann ('90, Taf. CLV. Fig. 7) 

 for reptiles ; by Dohrn ('91, Taf. V. Figg. 16 und 17) for Selachii; and by Sewert- 

 zoff ('95, Taf. V. Fig. 16) for Amphibia. 



