Still more characteristic of the Deciduates is the peculiar and very intimate connection between the chorion frondosum and the corresponding part of the mucous coat of the womb, which we must regard as a real coalescence of the two. The villi of the chorion push their branches into the blood-filled tissues of the coat of the uterus, and the vessels of each loop together so intimately that it is no longer possible to separate the foetal from the maternal placenta; they form henceforth a compact and apparently simple placenta. In consequence of this coalescence, a whole piece of the lining of the womb comes away at birth with the foetal membrane that is interlaced with it. This piece is called the "falling-away" membrane (decidua). It is also called the serous (spongy) membrane, because it is pierced like a sieve or sponge. All the higher Placentals that have this decidua are classed together as the "Deciduates." The tearing away of the decidua at birth naturally causes the mother to lose a quantity of blood, which does not happen in the Indecidua. The last part of the uterine coat has to be repaired by a new growth after birth in the Deciduates. (Cf. Figures 1.199 and 1.200.) In the various orders of the Deciduates, the placenta differs considerably both in outer form and internal structure. The extensive investigations of the last ten years have shown that there is more variation in these respects among the higher mammals than was formerly supposed. The physiological work of this important embryonic organ, the nutrition of the foetus during its long sojourn in the womb, is accomplished in the various groups of the Placentals by very different and sometimes very elaborate structures. They have lately been fully described by Hans Strahl. The phylogeny of the placenta has become more intelligible from the fact that we have found a number of transitional forms of it. Some of the Marsupials (Perameles) have the beginning of a placenta. In some of the Lemurs (Tarsius) a discoid placenta with decidua is developed.