THE LATER DEVELOPMENT OF THE FROG 



143 



They first appear in the anterior part of the body in the larva 

 of about 4 mm. As they grow outward they meet the dorsal 

 root, just beyond the ganglion, and pass thence in part to the 

 mesodermal myotomes, and in part 

 are distributed with the sympathetic 

 system. The two most anterior 

 myotomes are without spinal nerves; 

 these are occipital in position and 

 later disappear. 



C. THE SYMPATHETIC SYSTEM 



The development of the sympa- 

 thetic system is very imperfectly 

 known in the frog. The first defi- 

 nite indication of it appears in the 

 embryo of about 6 mm., as slight 

 collections of cells on the spinal 

 nerves about the level of the dorsal 

 aorta. From what is known here 

 and to be inferred from the condi- 

 tions in other lower vertebrates, it 

 may be said that these cell groups 

 are composed of elements from the 

 spinal ganglia and certain of the 

 posterior cranial ganglia. These , FIG. 46. Transverse section 



r through 8.6 mm. larva of R. 



Cells, COming from the ganglia, mi- esculenta, illustrating the rela- 



grate ventro-medially and form a ^J^S^SfSt 



pair of longitudinal Sympathetic COrds Dorsal aorta; c, spinal cord; 



, , , . , .' , , , d, dorsal (sensory, afferent) root 



along the Sides of the dorsal aorta O f spinal nerve; m, myotome; 



(Fig. 46). From the Cells Of these n ' notochord; r, ramus com- 



mumcans; sc, sympathetic cord; 

 COrds processes appear to grOW back sg, spinal ganglion; sn, spinal 



to the spinal ganglia forming the %?* $%?*' 

 rami communicantes, and also* pe- 

 ripherally to the visceral organs and surfaces. Along the 

 paths thus marked out fibers grow out from other spinal gang- 

 lion cells, and quite likely other cells migrate from the cord 



sc 



