578 



THE EMBRYOLOGY OF THE FROG 



The cells themselves migrate ventro-medially to form a pair of longi- 

 tudinal sympathetic cords, along each side of the dorsal aorta. It is from 

 these cords, then, that processes grow back to the spinal ganglia to form 

 the rami communicantes, as well as outwardly to the various organs and 

 surfaces. 



Other fibers from other spinal ganglia grow out and follow the paths 

 thus laid down for them, while cells from the sympathetic cord probably 

 also migrate to form the large sympathetic ganglia found in close con- 

 nection with the large blood vessels, and the thoracic and abdominal 

 viscera. The fact of the matter is that the sympathetic nervous system 



B. 



ao.c. 



iscA 



Fig. 337. 



A. Sympathetic nervous system of the Frog, ao, aortic arch ; 

 ao.c., common aorta ; isch, ischial nerve ; sp.sy., communicating 

 branches between the spinal and sympathetic nerves ; ay, the two 

 branches of the sympathetic system; 1-10, spinal nerves. (After 

 Meissner.) 



B. One-half of a transverse section of Ammocoetes, in the 

 head-region. Schematic. ao, aorta ; br.a., branchial branch of 

 nerves ; ch, notochord ; ep.br.pl., ganglion anlage which develops 

 where the epibranchial placode forms ; ect, ectoderm ; ent, ento- 

 derm ; lat.pl., lateral placode ; med.pl., medial placode ; med.obl., 

 medulla oblongata (hind-brain) ; mt, myotome ; s.c.a., subcutaneous 

 branch of the epibranchial nerve placode ; ap.a., spinal branch 

 running inward ; s.pl., lateral plate of mesoderm ; symp.g., sym- 

 pathetic ganglion. (After von Kupffer.) 



of the frog has not been worked out with any degree of thoroughness, 

 and we can only suppose many things from our knowledge of other forms 

 where more is known of this system. 



The ganglion of the III cranial nerve is sympathetic in character, 

 as other cranial nerves may be, but this must be left for future workers 

 to demonstrate. 



