. Elementary text-book of zoology. MAMMALIA. 497 features which might indicate an independent evolution of the allantoic placenta within the group. The epipubic bones have the same relations as those of the Prototheria, and the exact significance of the tooth- succession is not yet decided. The known facts are as follows:— The majority of the Metatheria retain the one set of teeth throughout life, with the single exception of the third upper and lower tooth on each side behind the canine, hence Fig. 342.—Diagram of Embryo of Perameles with FcETAL Membranes. (After Hill.) Yolk-sac Villi. Edge o
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. Elementary text-book of zoology. MAMMALIA. 497 features which might indicate an independent evolution of the allantoic placenta within the group. The epipubic bones have the same relations as those of the Prototheria, and the exact significance of the tooth- succession is not yet decided. The known facts are as follows:— The majority of the Metatheria retain the one set of teeth throughout life, with the single exception of the third upper and lower tooth on each side behind the canine, hence Fig. 342.—Diagram of Embryo of Perameles with FcETAL Membranes. (After Hill.) Yolk-sac Villi. Edge of Allantoic Placenta. AUantois with Villi. ^^. Amnion. Yolk-sac. Prokalymma. Note the allantoic villi. termed the third premolar. This tooth usually resembles the teeth behind it rather than those in front, and at some time (earlier or later according to the species) it falls out and is replaced by a permanent tooth. The next fact to note is the later discovery of a series of tooth-germs in the front of the jaw, which never cut the gum but are absorbed after reaching a certain stage. The deciduous premolar is said to rise in connection with these, and the most reasonable view seems to be to regard these M 33. Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original work.. Masterman, Arthur Thomas. Edinburgh, E. & S. Livingstone