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Updated: May 21, 2025
Their ancestors also had long tails and gills like the gilled Amphibia, as the tail and the gill-arches of the human embryo clearly show. For comparative anatomical and ontogenetic reasons, we must not seek these amphibian ancestors of ours as one would be inclined to do, perhaps among the tail-less Batrachia, but among the tailed lower Amphibia.
Of the four original gill-arches of the mammals the first lies between the primitive mouth and the first gill-cleft. At the outer surface of the latter is formed from the cellular matter of the corium, as covering or accessory bone, the permanent bony lower jaw.
Behind the lower jaw we find the hyoid bone at the root of the tongue, also formed from the gill-arches, and a part of the lower arches that have developed as "head-ribs" from the ventral side of the base of the cranium.
Examples of the second fact that individuals of higher classes or orders in former states of their embryonic development represent an organization which corresponds to the full-grown individuals of the lower classes are: the tail of the human embryo, the gill-arches of the embryos of reptilia, of birds, of mammalia, and of man.
As appendages of the vertebral column we have the ribs, and of the skull we have the hyoid bone, the lower jaw, and the other products of the gill-arches. The skeleton of the limbs or extremities is composed of two groups of parts the skeleton of the extremities proper and the zone-skeleton, which connects these with the vertebral column.
Among the important changes of the vertebrate organisation that marked the rise of the first Amniotes from salamandrine Amphibia during this period the following three are especially noteworthy: the entire disappearance of the water-breathing gills and the conversion of the gill-arches into other organs, the formation of the allantois or primitive urinary sac, and the development of the amnion.
One of the most salient characteristics of the Amniotes is the complete loss of the gills. The Protamniote itself must have entirely abandoned water-breathing. But we do not find in the embryos of the Amniotes any trace of gill-leaves, or of real respiratory organs on the gill-arches. It is very probable that the urinary bladder of the Dipneusts is the first structure of the allantois.
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