Page:Microscopicial researchers - Theodor Schwann - English Translation - 1947.pdf/36

STRUCTURE AND GROWTH

THE Chorda Dorsalis in the larve of frogs and fishes lies in, or in some instances, under the bodies of the vertebra, 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 thickest 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 recently 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 let 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 connexion 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. Müller has discovered in fishes, a cellular structure in its interior, surrounded 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