It accords with the widely extended principle of specialisation, that the glands over a certain space of the sack should have become more highly developed than the remainder; and they would then have formed a breast, but at first without a nipple, as we see in the Ornithorhyncus, at the base of the mammalian series.
Espezializazioaren hastapen zabal hedatuarekin ados dago zorroaren alderen bateko guruinak gainerakoak baino garatuago bihurtu izana; eta orduan ugatz bat formatuko zuten, nahiz eta hasieran ditibururik gabe, Ornitorrinkoan ikusten dugun moduan, ugaztunen sailaren oinarrian.