630 THE CLOACA. 



air-cells belonging to each minute lobe come in their further growth to 

 open into a common chamber. Before the lungs assume their function 

 the embryonic air-cells undergo a considerable dilatation. 



The 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, cartila- 

 ginous, vascular, and other connective tissues of the trachea! and 

 bronchial walls. 



There can be no doubt that the lungs and air-bladder are homo- 

 logous structures, and the very interesting memoir of Eisig on the 

 air-bladder of the Chsetopoda 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 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 itiay be defined as the dilated 

 portion of the alimentary tract which receives the genital and urinary 

 ducts and opens externally by the proctodveum. 



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. 



1 H. Eisig, " Ueb. cl. Vorkommen ernes schwimmblasenahnlichen Organs bei Anne- 

 liden." Mittheil. a. iL ~.ool. Station ~. K cupel, Vol. n. 1881. 



