10 STRUCTURE AND GROWTH 
SECTION I. 
ON THE STRUCTURE AND GROWTH OF THE CHORDA DORSALIS 
AND CARTILAGE. 
1. Chorda Dorsalis. 
Tux Chorda Dorsalis in the larve of frogs and fishes les 
in, or in some instances, under the bodies of the vertebrae, and 
is continued behind the coccyx, through the whole length of 
the tail. It is imclosed 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- 
cently killed animals, but is best obtamed 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. Miller 
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 by its 
own proper membrane. The cells vary much in size, being 
