Vertehrata. 



343 



■Ex. 



and some others), whilst the former are limited to Fish and the 

 larvae of Amphibia. The gills usually consist of very vascular 

 laminae arranged in a single series on the sides of the gill-clefts; 

 the latter are a series of large lateral slits, which lie closely 

 behind each other as perfora- 

 tions in the wall of the 

 oral-cavity. The clefts are 

 separated by septa, in which 

 lie the visceral arches men- 

 tioned above {see also Pisces) . 

 It is of great interest that in 

 the embryos of all higher 

 Vertebrata (Reptilia, Aves, 

 Mammalia), which at no time 

 of their lives breathe by gills, 

 similar visceral clefts occur ; 

 they do not, however, bear 

 gill filaments, and they close 

 again later. 



The lungs arise as an 

 unpaired evagination of the 

 alimentary canal, at the junc- 

 tion of the buccal-cavity and 

 oesophagus. During further 

 development, this evagination 

 does not usually remain simple, 

 but divides into two sacs, right 

 and left, communicating with 

 the alimentary canal by a 

 common tube. Bach sac, in 

 the simplest case, grows into 

 a large, thin-walled bag, the 

 wall of which is richly sup- 

 plied with blood vessels {e.g., in the common Newt). Usually, 

 however, the inner surface of the pulmonary sac becomes increased by 

 evaginations, which remain united by connective tissue, so that, the 

 outer surface of the lung appears almost smooth. In some instances 

 {e.g., the Frog) each lung contains a large central cavity and the 

 evaginations are short ; in others, the latter become longer and 

 branching, whilst the central cavity is smaller (Reptilia) ; in the 

 highest condition (in the Mammalia) the original sac is so much 

 branched that it forms an arborescent organ, the finest branches of 

 which end in tiny thin-walled vesicles. Over the walls of these 

 minute sacs a fine capillary network is spread, whilst the trunk and 

 the larger branches become thick-walled and firm, forming a kind of 

 skeleton for the rest of the lung. Further, both larger and smaller 



Fig. 283. Anterior portion of a Chick 

 embryo (four days' inouhation). ANr incipient 

 lachrymal canal (still a groove), Ex incipient left 

 limb, Gh auditory vesicle, H fore brain, Sz heart, 

 Lb liver, Ls lens of eye, Liv body wall, M mouth, 

 Mg rudiment of stomach, Mh mid-brain. Ok upper 

 jaw, Sg olfactory pit, Sp first visceral 

 cleft behind which three others may 

 be observed, C7fc lower jaw. — After His. 



