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Updated: June 22, 2025
This is not so, for like the adult fish, the larval tadpole, and the embryo chick, an embryo of a cat or a man is at one time constructed with a series of gill-clefts and with blood-vessels and skeletal supports of fishlike nature that are everywhere associated with gills.
I have already pointed out that this modification is distinctive of the Vertebrates and Tunicates. The phylogenetic appearance of the gill-clefts indicates the commencement of a new epoch in the stem-history of the Vertebrates. In the further ontogenetic development of the alimentary canal in the human embryo the appearance of the gill-clefts is the most important process.
The branchial or head-gut of the Ascidia is small at first, and opens directly outwards only by a couple of lateral ducts or gill-clefts a permanent arrangement in the Copelata. The gill-clefts are developed in the same way as in the Amphioxus. As their number greatly increases we get a large gill-crate, pierced like lattice work.
The agreement in the structure of the branchial gut of the Enteropneusts, Tunicates, and Vertebrates was first recognised by Gegenbaur ; it is the more significant as at first we find only a couple of gill-clefts in the young animals of all three groups; the number gradually increases.
None the less, if the transition from a gilled tadpole to the adult with lungs means an evolution of amphibia from fishlike ancestors, then the change of a chick embryo with gill-clefts into the fledgling without them is most reasonably interpreted as proof that birds as well as amphibia have had ancestors as simple as fishes.
It is the same with the human mouth and that of the Craniotes generally. The secondary formation of the mouth in the Chordonia is probably connected with the development of the gill-clefts which are formed in the gut-wall immediately behind the mouth. In this way the anterior section of the gut is converted into a respiratory organ.
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