344 



COMPARATIVE ANATOMY 



each olfactory pit to the corner of the mouth. Later the groove deepens, 

 its edges meet and fuse together and convert the groove into a tubular 

 passage which connects the pit with the mouth cavity (Fig. 471). 



The development of the narial passage in the human embryo is 

 slightly different. In the month-old embryo a similar nasobuccal groove 

 makes its appearance. The narial passage, however, is not formed by 



the closure of this groove, but by the 

 backward extension of the epithelium 

 of the olfactory pit, which thus 

 acquires a secondary connexion with 

 the mouth (Fig. 473). 



That this method of formation 

 of the narial passage is a modification 

 of the method found in lower verte- 

 brates is indicated by the not infre- 

 quent occurrence of hare-lip in 

 infants. Hare-lip is best explained 

 as a reversion to the more primitive 

 mode of development of the narial 

 passage from a groove. The hare- 

 lip arises when the groove fails to 

 close over. In some infants the 

 defect in growth is so extensive as to 

 cause a perforate or "cleft" palate 

 and to necessitate surgical operation. 

 History of the Pulmonary 

 System. Invertebrates have no 

 organs comparable with the human 

 pulmonary system, the so-called 

 lungs of pulmonate molluscs being 

 modifications of the mantle and not 

 outgrowths from the aUmentary 

 canal. Opinions are divided as to 

 the origin of the lungs. According to some, a pair of gill pouches which 

 failed to reach the skin have been converted into lungs. Others suppose 

 that lungs have evolved from the air bladder of fishes. Some seek to 

 reconcile these two divergent opinions by asserting that the air bladder is 

 itself derived from a pair of modified gill pouches. 



Goette (1875) was the first to suggest that lungs are modified gill 

 pouches, on the ground that in some amphibian embryos the lungs develop 

 from a pair of posterior endodermal pouches in series with the gill pouches. 

 A number of observers have confirmed this observation and reached the 

 same conclusion. In support of Goette's hypothesis is the fact that the 

 pulmonary arteries develop from the sixth pair of aortic arches. Further- 



FiG. 288. — Diagrams of air bladder 

 in fishes. A, Physostomous fishes; B, 

 Lepidosteus and Amia; C, Erythrinus; 

 D, Ceratodus. The air bladder of the 

 Crossopterygian fish, Polypterus, is, like 

 the lungs of amphibians, bilobed and con- 

 nected with the floor of the pharynx. 

 (From Kingsley's "Comparative Anat- 

 omy of Vertebrates," after Dean.) 



