It follows from the above account that the whole pulmonary structure is the result of the growth by budding of a system of branched hypoblastic tubes in the midst of a mass of mesoblastic tissue, the hypoblastic elements giving rise to the epithelium of the tubes, and the mesoblast providing the elastic, muscular, cartilaginous, vascular, and other connective tissues of the tracheal and bronchial walls.
There can be no doubt that the lungs and air-bladder are homologous structures, and the very interesting memoir of Eisig on the air-bladder of the Chætopoda[280] shews it to be highly probable that they are the divergent modifications of a primitive organ, which served as a reservoir for gas secreted in the alimentary tract, the gas in question being probably employed for respiration when, for any reason, ordinary respiration by the gills was insufficient.
Such an organ might easily become either purely respiratory, receiving its air from the exterior, and so form a true lung; or mainly hydrostatic, forming an air-bladder, as in Ganoidei and Teleostei.
It is probable that in the Elasmobranchii the air-bladder has become aborted, and the organ discovered by Micklucho-Maclay may perhaps be a last remnant of it.
The middle division of the mesenteron. The middle division of the mesenteron, forming the intestinal and cloacal region, is primitively a straight tube, the intestinal region of which in most Vertebrate embryos is open below to the yolk-sack.
Cloaca. In the Elasmobranchii, the embryos of which probably retain a very primitive condition of the mesenteron, this region is not at first sharply separated from the postanal section behind. Opposite the point where the anus will eventually appear a dilatation of the mesenteron arises, which comes in contact with the external skin ([fig. 28] E, an). This dilatation becomes the hypoblastic section of the cloaca. It communicates behind with the postanal gut ([fig. 424] D), and in front with the intestine; and may be defined as the dilated portion of the alimentary tract which receives the genital and urinary ducts and opens externally by the proctodæum.
In Acipenser and Amphibia the cloacal region is indicated as a ventral diverticulum of the mesenteron even before the closure of the blastopore. It is shewn in the Amphibia at an early stage in [fig. 73], and at a later period, when in contact with the skin at the point where the anal invagination is about to appear, in [fig. 420].
Fig. 420. Longitudinal section through an advanced embryo of Bombinator. (After Götte.)
m. mouth; an. anus; l. liver; ne. neurenteric canal; mc. medullary canal; ch. notochord; pn. pineal gland.