766 THE CLOACA. 



Before the lungs assume their function the embryonic air-cells undergo a 

 considerable dilatation. 



77^6' trachea and larynx. The development of the trachea and larynx 

 does not require any detailed description. The larynx is formed as a simple 

 dilatation of the trachea. The cartilaginous structures of the larynx are of 

 the same nature as those of the trachea. 



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 Chaetopoda 1 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 even- 



1 II. Eisig, " Ueb. d. Vorkommen eines schwimmblasenahnlichen Organs bei 

 Anneliden." Mittheil. a. d. zool. Station z. Ncapcl, Vol. n. 1881. 



