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Updated: May 17, 2025
The differences between the Mammals and the reptiles and birds are so important and characteristic that we can assume with complete confidence this division of the vertebrate stem at the commencement of the development of the Amniotes.
Taking all these peculiarities of the Amniotes together, it is impossible to doubt that all the animals of this group all reptiles, birds, and mammals have a common origin, and form a single blood-related stem. Our own race belongs to this stem.
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.
Homoeosaurus pulchellus, a Jurassic proreptile from Kehlheim. The instructive group of the Permian Tocosauria, the common root from which the divergent stems of the Sauropsids and mammals have issued, merits our particular attention as the stem-group of all the Amniotes.
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.
In its earliest form the heart is DOUBLE, as recent observation has shown, in all the Amniotes, and the simple spindle-shaped cardiac tube, which we took as our starting-point, is only formed at a later stage, when the two lateral tubes move backwards, touch each other, and at last combine in the middle line.
This is only so as a passing phase of the early embryonic life in the three higher classes of Vertebrates, the Amniotes. They represent the third and last generation of the vertebrate kidneys. Here a simple tube, the secondary renal duct, develops, near the point of its entry into the cloaca; and this tube grows considerably forward.
There the primitive urine accumulates, and thus the allantois acts as bladder or urinary sac in the embryos of man and the other Amniotes. Thus it is a product of the visceral layer, whereas the primitive kidneys are a product of the middle layer.
The great interest of the natural history of the Amphibia consists especially in their intermediate position between the lower and higher Vertebrates. The lower Amphibia approach very closely to the Dipneusta in their whole organisation, live mainly in the water, and breathe by gills; but the higher Amphibia are just as close to the Amniotes, live mainly on land, and breathe by lungs.
The fore-end of the medullary tube expands into a vesicle and forms the brain, which soon divides into five cerebral vesicles. In the sides of it appear the three higher sense-organs: nose, eyes, and auditory vesicles. No jaws, limbs, or floating bladder. The ancestors of these Amniotes develop an amnion and allantois, and gradually assume the mammal, and finally the specifically human, form.
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