22 ON THE NERVOUS SYSTEM IV. 



winter in the same condition, the metamorphosis occurring during the following 

 spring or summer. 



Until the legs begin to be developed, the chord presents the form of an extremely 

 elongated cone, and the bulgings, as was noticed by Serres, are developed simulta- 

 neously with the legs. I have not, however, been able to confirm the statement 

 made by him, and repeated by others, that the caudal portion of the chord is short- 

 ened as the legs and the bulgings are developed. According to my observations, 

 no shortening takes place until the absorption of the tail commences, and this 

 happens after the bulgings are formed and the legs have acquired their growth. 

 The whole of the caudal prolongation, however, is not absorbed, a portion being 

 persistent, and eventually becoming enveloped by the elongated coccyx. 



The caudal portion of the chord in the tadpole is so much reduced in size, that 

 it affords an unusually good opportunity for microscopic examination. It is much 

 flattened, has no fissures, is of an opaque white on the sides, but in the centre a 

 transparent band is visible through its whole length. Examined with a power of 

 one hundred diameters, or even less (Plate I. Figs. 16, 17), it is easily resolved 

 into nerve tubes and cells, the first forming the opaque lateral bands, and the latter 

 a transparent central stripe. The cells are like those described as existing in 

 the brain, spheroidal, filled with granules, the walls attenuated and easily rup- 

 tured. None of them are caudate, and no direct connection was noticed between 

 nerve tubes and cells, the former being almost wholly longitudinal, running with 

 great regularity parallel to each other, and giving off nerves to the lateral muscles 

 which form so large a portion of the tail. As the eye traverses the chord from the 

 apex towards the brain, there will be seen, as we approach the former, here and 

 there a nerve tube running more or less obliquely across from one side to the 

 other ; a little farther forwards the tubes become more numerous, until at length 

 they are found very abundant in the posterior part of the trunk. Over the trans- 

 parent central portion of the chord there may be seen, with a very high power, 

 very minute longitudinal fibres (Plate I. Figs. 15, 16), which appear to be solid, 

 and have not the double outline found in nerve tubes caused by the substance 

 of Schwann. 



From the description just given, it Avill be seen that the chord gradually becomes 

 more simple in the caudal portion, that transverse fibres gradually become less 

 numerous, and are eventually omitted, and that the nerve tubes and cells, as else- 

 where, have no direct connection. The lateral strands of nerve tubes appear to 

 be continuous with the white columns of the chord proper, and if they anywhere 

 intermix with the nerve cells, it must be in some other place than in the caudal 

 portion. 



In one particular, then, the caudal prolongation of the chord in the tadpole 

 differs in structure from that of the true chord, in having the nerve roots all 

 ascending towards the dorsal region of the chord, and none of them passing 

 among the cells of the central portion opposite to the points at which they are 

 attached. 



