EMBRYOLOGY OF TADPOLE AND CHICK 



577 



of the medulla oblongata which send out processes called axons, which 

 leave the medulla in close association with the sensory roots already de- 

 scribed. These are then distributed with the branches passing posterior 

 to the gill clefts. 



The III cranial nerve is the first of the remaining III, IV, and VI 

 to appear, although all three of these form later than the ones discussed 

 above, that is, they form when the tadpole is five to six millimeters in 

 length. 



The III is called the oculo-motor, the IV the trochlear, and the VI 

 the abducens. All are motor nerves, which innervate the muscles of the 

 eye-ball. 



The I cranial nerve is the purely sensory olfactory nerve and the II 

 is likewise a purely sensory nerve, namely, the optic. 



THE SPINAL NERVES 



These nerves, unlike the cranial nerves, are related to the somites, 

 and not to the visceral clefts, and there are no placodes connected with 

 them. 



The two most anterior myotomes 

 do not have spinal nerves connected 

 with them, and the myotomes soon dis- 

 appear, but the segments formed in the 

 neural crests, posterior to the head re- 

 gion (with the exception of the two 

 just mentioned), have cell processes 

 grow out into the cord to form the dor- 

 sal root of the spinal nerves, while 

 others grow away from the cord to 

 form the peripheral strands which are 

 distributed to the skin and other 

 sensory surfaces. 



The ventral root of the spinal 

 nerve is formed by outgrowths or 

 axons from the neuroblasts on the ven- 

 tral side of the cord, and appear when 

 the tadpole is about four millimeters 

 in length. These then meet the dorsal 

 root a little distance beyond the ganglion, and pass partly to the meso- 

 dermal myotomes and partly to the sympathetic system. 



THE SYMPATHETIC SYSTEM (Fig. 337) 



When the tadpole is about six millimeters in length, one may see a 

 slight collection of cells on the spinal nerves at about the level of the 

 dorsal aorta. From our study of the sympathetic system in higher ani- 

 mals, we assume that these cell-groups are composed of elements from 

 the spinal ganglia, and from some of the posterior cranial ganglia. ,. t 



Fig. 336. 



Schematic arrangement to show the 

 composition of the central nerve roots in 

 shark fins. The motor fibers run to the 

 muscles, and each motor spinal root is 

 made up of the fibers of three spinal seg- 

 ments. NI, II, III. IV, V, Neuromeres; 

 N2, N3, N4, corresponding motor roots ; 

 M1-M5, Myomeres; 1-10 Divisions of 

 Mydmeres. (From Rabl.) 



