730 ANATOMY OF VERTEBRATES. 



with the substance lining the human uterus, but is firmer, and to 

 no part can the term • mucous membrane ' be correctly applied. 

 The placental disc in the Tenrec is subcircular, thickest at the 

 periphery ; in all other essential points it agrees with the rest of 

 its order. The main peculiarity of Centetes is its multiparity. 1 

 From four to six foetuses may be brought forth by the Hedgehog: 

 from twelve to twenty by the Tenrec. The shape of the placenta 

 changes in the course of utero-gestation in Insectivora. When 

 the embryo Hedgehog is from half an inch to an inch in length, 

 it is enclosed in a cup-shaped placenta, as in a nest : this is sub- 

 sequently spread out and flattened by the growth of the foetus, 

 and converted into a thin, shallow discoid plate, with its concavity 

 applied to the back of the embryo, and with the central part of 

 its convex surface attached to the uterus : the ' button ' lies flat 

 upon the maternal portion, and is attached by a wider surface than 

 in the Guinea-pig. In the Mole the placenta is a circular disc at 

 the early period of gestation, and subsequently becomes an oblong 

 flat band, with its long axis parallel to that of the foetus : the linear 

 tract of the uterine surface to which the placenta is attached shows 

 a fine areolar structure, penetrated by the foetal placentary fila- 

 ments, which are often brought away, as in the Rat, distinct from 

 the maternal structure, like the foetal cotyledon in the Cow. 2 In 

 the Bat ( Vespertdio noctula), the placenta has the form of an obtuse 

 cone. In all the foregoing insectivorous mammals the vitellicle 

 is large. But, in a frugivorous Bat (Pteropus medius, Temminck), 

 I found the vitellicle shrunk to a reniform, compactly folded body, 

 which lay in the concavity of the placenta, between it and the 

 allantois : the placenta was subcircular, discoid, slightly concave 

 towards the foetus, proportionally more convex towards the 

 uterus. The foetal villi are long, delicate, and branched, giving a 

 flocculent appearance to the small portion of the centre of the 

 disc by which the foetal placenta is attached to the womb. 



Volant Insectivora, in relation to the exigencies of flight, are 

 commonly uniparous. The uterus of Vesper tilio emarg hiatus , 



1 The chief of the alleged ' points of difference ' of the Tenrec's placental structures 

 from those of other Insectivora, in • the absence of a yelk-sac, of the allantois as a 

 distinct sac, and of any membrane either decidual or chorionic, on the exterior of the 

 amnios' (ccxxvn". p. 291), depend (granting the competency of the observer) on the 

 admitted state of decomposition of the specimen described (p. 287): the 'absence of a 

 yelk-sac,' moreover, would point from, rather than toward, ' marsupial affinities,' in- 

 asmuch as the embryo of the Kangaroo is chiefly remarkable for the large size of such 

 sac, or ' vitellicle.' 



2 This never happens in the quadrumanous placenta ; and the difference is not 

 affected by showing that, prior to severance, the foetal placenta is combined with 

 maternal vessels in the Rat, Mole, &c. 



