. Text-book of embryology. Embryology. Ill AIK-BLADDEE 173 situated laterally, balancing one. another, while farther back where only the right lung is present this shifts towards the mesial plane until it is symmetrical about that plane, lying in the dorsal mesentery (Fig. 98, A and B). Evolution of the Air-Bladder.—The facts that have been enunciated above, with regard to the development of the lung in Dipnoan and Crossopterygian fishes, are of much morphological interest. When pieced together with what has been said regarding the development of the air-bladder of Teleostean fishes they affor
. Text-book of embryology. Embryology. Ill AIK-BLADDEE 173 situated laterally, balancing one. another, while farther back where only the right lung is present this shifts towards the mesial plane until it is symmetrical about that plane, lying in the dorsal mesentery (Fig. 98, A and B). Evolution of the Air-Bladder.—The facts that have been enunciated above, with regard to the development of the lung in Dipnoan and Crossopterygian fishes, are of much morphological interest. When pieced together with what has been said regarding the development of the air-bladder of Teleostean fishes they afford data from which the evolutionary history of the Teleostean air- bladder can be traced out with a high degree of probability. That history may be stated in a few words to have probably been as follows: 1. The primitive condition was that of a lung, communi- cating with the pharynx by a ven- trally placed glottis —for we have seen that the embryonic rudiment of the organ in the most archaic forms pos- sessing it is a typical lung-rudiment. 2. The organ became bilobed, growing back into a right lung and a left lung. 3. In the forms which took to a purely swimming existence, and became specialized in the direction of adaptation to this, there came about an asymmetry of the lungs, the right lung increas- ing and the left lung diminishing. Why this should have happened is not yet absolutely certain : it may probably have been in adapta- tion to active movements of lateral flexure, for we see the same thing taking place in Gymnophiona, Snakes and Snake-like Lizards. That it has been the right rather than the left lung which has increased in size, is probably correlated with the rotation of this region of the alimentary canal in a counter-clockwise direction as seen from behind (see p. 168) which would tend to interfere more with the circulation through the left lung than with that through the right, by lengthening the course of the left pulmonary artery. Steps. Fig. 98.—Sectio
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