Jones, Development of the Sympathetic. 125 



fifth, sixth, seventh, and eighth nerves, there is also an incipient 

 one on the ninth. 



Twenty-one Millimeter Stage. — In a toad 21 mm. long, the 

 sympathetic cord is continuous from the vagus ganglion back to 

 the region between the ninth and tenth nerves. But it is to be 

 borne in mind that the development back of the second nerve 

 is different from that in front of this point. The ridge has 

 mostly disappeared. A considerable portion of it remains, how- 

 ever, between the fourth and fifth nerves and smaller parts, pos- 

 teriorly. Compare Fig. 10, Ri., where the ridge is well devel- 

 oped, with Fig. II, Ri., in the region of the eighth and ninth 

 nerves, where the ridge has almost completely disappeared. 



Between the vagus ganglion and the second spinal nerve, 

 the sympathetic cord is removed a considerable distance dor- 

 sally from the aorta. (Cf. Fig. 2, Sy., where it lies almost up- 

 on the aorta, Ao. 2.) The mass of cells anterior to the kidney, 

 with which the ridge was connected in earlier stages (Fig's. 

 I and 7, Ma.), is still present, but is flattened posteriorly, where 

 it occurs as a horizontal plate-like mass of cells extending later- 

 ally from the aorta. As the ridge disappears, the sympathetic 

 cord is left alone, large and distinct, lying most of the way upon 

 this mass of cells but not continuous with it and removed later- 

 ally a considerable distance from the aorta. In the region of 

 the kidney, the cords lies on the dorsal side of this organ. The 

 sympathetic cords of the two sides can be traced back as far as a 

 point midway between the ninth and tenth nerves, where they 

 unite underneath the aorta. 



Beginnings of rami are found in connection with the first 

 and third nerves ; a small one is seen on the second, while rath- 

 er distinct ones appear on the fourth, fifth, sixth, seventh, eighth, 

 and ninth nerves (Fig. ii, R.). The sympathetic cord is sur- 

 rounded by a well-developed membrane, throughout most of its 

 extent. The cord is composed now not of cells alone, for most 

 of the way it shows fibrous structure also. Furthermore, it is 

 better developed in the immediate region of the nerves than it 

 is between them (Fig. 11, Sy.), a condition only slightly no- 

 ticeable in earlier stages. Between the nerves, the cord is rela- 



