THE ORGANS OF THE OUTER GERM-LAYER. 



451 



in front and behind, whereas in the segments that are cut more 

 dorsally, near the origin of the ganglia from the neural tube, the 

 fundaments are still connected with one 

 another. This connection appears to be 

 most conspicuously developed and most per- 

 sistent in the case of the Selachians ; it 

 has been called the longitudinal commis- 

 sure by BALFOUR. Outside the ganglia are 

 found the primitive segments (mp, mp'), each 

 of which at this time still exhibits within 

 it a narrow fissure. 



In a monographic treatment of the peripheral 

 nervous system BEARD differs from the preceding 

 account, in which BALFOUR, KOLLIKER, BABL, 

 HENSEN, SAGEMEHL, KASTSCHENKO, and others 

 agree. He believes that the fundaments of the 

 ganglia arise, not out of the neural tube, but out 

 of the deeper cell-layers of the adjacent part of 

 the outer germ-layer. He finds that they are from 

 the beginning separated from each other and seg- 

 mentally arranged. According to him, moreover, 

 they make their appearance earlier than is stated 

 in the preceding account ; for they are already 

 recognisable as especially thickened places in the 

 outer germ-layer at the light and left of the neural 

 plate when the latter first begins to be bent inward. 



Subsequently, upon the closure of the neural tube, the ganglionic cells come 

 to lie between the raphe and the primitive epidermis. From here they grow 

 down ventrally at the sides of the brain and spinal cord. 



BEARD approximates in his results the conception first expressed and 

 subsequently maintained by His. For His derives the ganglionic ridge, not 

 from the raphe of the neural tube, but from a neighboring part of the 

 outer germ-layer, which he names intermediate cord (Zwischenstrang). The 

 accuracy of BEARD'S description is, however, positively denied by RABL and 

 KASTSCHENKO. 



Different views are entertained concerning the further changes 

 which take place in the fundaments of the spinal ganglia : 



According to His and SAGEMEHL the separate ganglionic funda- 

 ments are completely detached from the neural tube, and for a time 

 lie at the side of it without any connection with it whatever. 

 Secondarily a union is again established, through the development 

 of the dorsal nerve-roots, by the formation of nerve-fibrillse, which 

 either grow out from the spinal cord into the ganglion, or from the 

 .ganglion into the spinal cord, or in both directions. SAGEMEHL 



Fig. 259. Frontal section of 

 a Lizard embryo, after 

 SAGEMEHL. 



rm, Spinal cord ; spk, neural 

 ridge with thickenings that 

 are converted into the spinal 

 ganglia; mp', the part of 

 the primitive segment that 

 produces the muscle-plate ; 

 mp, outer layer of the 

 primitive segment. 



