In Ascidians the respiratory sack is homologous with the respiratory tract of Amphioxus.
The details of the development of the branchial clefts in the different groups of Vertebrata have already been described in the systematic part of this work.
In all the Ichthyopsida the walls of a certain number of clefts become folded; and in the mesoblast within these folds a rich capillary network, receiving its blood from the branchial arteries, becomes established. These folds constitute the true internal gills.
In addition to internal gills external branchial processes covered by epiblast are placed on certain of the visceral arches in the larva of Polypterus, Protopterus and many Amphibia. The external gills have probably no genetic connection with the internal gills.
The so-called external gills of the embryos of Elasmobranchii are merely internal gills prolonged outwards through the gill clefts.
The posterior part of the primitive respiratory division of the mesenteron becomes, in all the higher Vertebrata, the œsophagus and stomach. With reference to the development of these parts the only point worth especially noting is the fact that in Elasmobranchii and Teleostei their lumen, though present in very young embryos, becomes at a later stage completely filled up, and thus the alimentary tract in the regions of the œsophagus and stomach becomes a solid cord of cells ([fig. 23] A, œs): as already suggested (p. [61]) it seems not impossible that this feature may be connected with the fact that the œsophageal region of the throat was at one time perforated by gill clefts.
In addition to the gills two important organs, viz. the thyroid body and the lungs, take their origin from the respiratory region of the alimentary tract.
Fig. 414. Diagrammatic vertical section of a just-hatched larva of Petromyzon. (From Gegenbaur; after Calberla.)
o. mouth; o´. olfactory pit; v. septum between stomodæum and mesenteron; h. thyroid involution; n. spinal cord; ch. notochord; c. heart; a. auditory vesicle.