Comparative embryology shows interesting differences in the details of development of the gill-clefts, and in their ultimate fate. A common variation is that the individual cleft is at first solid and only secondarily becomes hollow : this is the case where the pharynx is in its early stages solid and yolk-laden (teleostean, fish, lungfish, amphibians).
Visceral cleft I. develops in 411 gnathostomes characteristic differences from the others. In elasmobranchs its respiratory lamellae are reduced to the vestigial pseudobranch on its anterior wall, its function being, as in crossopterygians and sturgeons, that of a mere passage for the water of respiration. In Dipnoi its outer ectodermal end forms a sensory organ (organ of Pinkus) embedded in the side of the head. In these and in all terrestrial vertebrates, it has lost its communication with the exterior. In anurous amphibians and in all amniotes it expands towards its outer end into a wide tympanic cavity lying immediately under the skin so as to allow a wide flat area of the latter to vibrate freely (tympanic membrane or ear-drum). The pharyngeal por tion of the cleft remains as the narrow Eustachian tube, provid ing a means of keeping the air-pressure equal on the two sides of the ear-drum.
While the presence of pharyngeal visceral clefts constitutes one of the most striking vertebrate characteristics, the evidence of comparative anatomy clearly indicates that the series of clefts is undergoing a gradual process of reduction. This is shown by the diminishing number of clefts present in the series Amphioxus cyclostomes (up to 14 in Bdellostorna)—Gnathostomata. Em bryology shows us this process of reduction actually at work. In various elasmobranch embryos vestigial pouches appear behind those which actually develop into clefts. In teleosts the vestigial spiracular rudiment flattens out while its pseudobranch remains visible on the inner surface of the operculum, thus appearing in the adult as if it belonged to visceral cleft II. The operculum of fishes above elasmobranchs is simply the exaggerated valvular flap formed by the outer edge of visceral arch II. (hyoid) which grows back to cover the visceral clefts further back in the series.
The thyroid, an equally characteristic development of the vertebrate pharynx, arises as a mid-ventral downgrowth of the pharyngeal floor about the level of the hyoid arch. This rudiment, arising either as a hollow pouch or as a solid structure which develops a cavity secondarily, soon becomes isolated from the pharynx as a closed vesicle, and this in turn becomes subdivided up into a multitude of little spherical sacs of endoderm sepa rated by mesenchyme, in which there arises a rich network of blood-spaces. The endodermal epithelium is glandular, producing
a clear colloid secretion which distends the numerous rounded vesicles. In its later development the thyroid differs in different vertebrates. It may, as in teleostean fishes, become diffuse and no longer recognizable as a compact organ, while in tetrapods it retains its compact form but becomes more or less completely separated into a right and left lobe.
The clue to the evolutionary history of the thyroid is given by the embryology of the lamprey (Petromyzon), in the larva of which it is recognizable as an endostyle, an organ known also in Amphzoxus and Tunicates.
The Lung, a characteristic feature in the main groups of Vertebrata above the elasmobranchs, arises normally from a rudiment very similar to that of the thyroid, only situated further back, about the hinder limit of the pharynx. Normally unpaired at first, the rudiment soon divides into right and left branches. In simple urodeles each lung remains a thin-walled membranous sac, but in other tetrapods increase in area of the endodermal respiratory lining is brought about by its bulging out into more and more complicated recesses, culminating in large reptiles and in mammals in a spongy texture. In birds, the endoderm-lined cavities cf the lung become converted into fine tubular channels (air-capillaries) interwoven with the blood-capillaries and con stituting the most highly evolved respiratory organ known. To enable it to function, a bellows-like arrangement is formed by pocket-like outgrowths of the lung-wall which become greatly dilated and constitute the air-sacs. Portions of these grow out into the substance of the bones, replacing the bone-marrow, while others extending in among the muscles of flight provide a mechanism whereby air is automatically passed in and out through the air-capillaries during flight. In Polypterus, the most archaic teleostome, the left lung lags behind in development, while the right grows actively and in its posterior portion takes up a medio-dorsal position. This is related to the hydrostatic function of the air-filled lung and is of great interest as indicating, when taken in conjunction with the development of the lung of Dipnoi, that the air-bladder of the modern teleostean fish is simply the persistent right lung, the left having completely dis appeared The post-pharyngeal portion of the alimentary canal forms the digestive tube and in different vertebrates undergoes varying degrees of differentiation into distinct parts. The more archaic gnathostomes elasmobranchs, crossopterygians, Dipnoi — are characterized by having a very short straight intestine provided with a spiral valve, and embryology shows that this is preceded by a stage in which the endodermal tube is relatively elongated and coiled into a tight corkscrew spiral, the turns of the spiral being later cemented together and hidden in an enveloping sheath of connective tissue.