10 STRUCTURE AND GROWTH 



SECTION I. 



ON THE STRUCTURE AND GROWTH OF THE CHORDA DORSALIS 



AND CARTILAGE. 



1. Chorda Dor salts. 



The Chorda Dorsalis in the larva? of frogs and fishes lies 

 in, or in some instances, under the bodies of the vertebras, and 

 is continued behind the coccyx, through the whole length of 

 the tail. It is inclosed by a firm sheath, and forms a spindle- 

 like, consistent, gelatiniform, transparent cord, which is thick- 

 est at the commencement of the tail, and thence gradually 

 diminishes in each direction, both towards the skull and the 

 point of the tail. It cannot well be separated entire in re- 

 centlv killed animals, but is best obtained from them in the 

 form of delicate transverse sections. If the animal be placed 

 in water for twenty-four hours or longer after death, and the 

 tail then severed from the body at their point of junction, the 

 chorda dorsalis may be entirely pressed out, by gently scraping 

 along its course from the point of the tail, or from the head, 

 towards the wound. As this does not succeed if the animal be 

 allowed to lie out of water for the same period after death, the 

 easier separableness appears to depend upon a penetration of the 

 water between the chorda dorsalis and its sheath ; the firmer con- 

 nexion of it in the fresh condition, however, only upon a more 

 close contact, or wedging in of the chorda dorsalis, and not upon 

 a vascular connexion, for I do not suppose that it contains any 

 vessels. Microscopically examined, it exhibits, as J. Miiller 

 has discovered in fishes, a cellular structure in its interior, sur- 

 rounded externally by a proportionately thin cortical substance 

 (rinde), which is beset with scattered granules. The interior 

 exactly resembles the parenchymatous cellular tissue of plants. 

 (See plate I, fig. 4.) It is readily seen, especially at the point 

 of contact of three cells, that each one is surrounded bv its 

 own proper membrane. The cells vary much in size, being 



