. Comparative anatomy. Anatomy, Comparative. 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
. Comparative anatomy. Anatomy, Comparative. 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 conf
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