The second type of differentiation from the primitive form of discoidal placenta is illustrated by the Lemuridæ, the Suidæ, and Manis. In all these cases the area of the placental villi appears to have increased so as to cover nearly the whole subzonal membrane, without the villi increasing to any great extent in complexity. From the diffused placenta covering the whole surface of the chorion, differentiations appear to have taken place in various directions. The _metadiscoidal_ placenta of Man and Apes, from its mode of ontogeny (p. 248), is clearly derived from a diffused placenta--very probably similar to that of Lemurs--by a concentration of the foetal villi, which are originally spread over the whole chorion, to a disc-shaped area, and by an increase in their arborescence. The polycotyledonary forms of placenta are due to similar concentrations of the foetal villi of an originally diffused placenta. In the Edentata we have a group with very varying types of placenta. Very probably these may all be differentiations within the group itself from a diffused placenta, such as that found in Manis. The zonary placenta of Orycteropus is capable of being easily derived from that of Manis, by the disappearance of the foetal villi at the two poles of the ovum. The small size of the umbilical vesicle in Orycteropus indicates that its discoidal placenta is not, like that in Carnivora, directly derived from a type with both allantoic and umbilical vascularization of the chorion. The discoidal and dome-shaped placentæ of the Armadilloes, Myrmecophaga, and the Sloths may easily have been formed from a diffused placenta, just as the discoidal placenta of the Simiadæ and Anthropidæ appears to have been formed from a diffused placenta like that of the Lemuridæ. The presence of zonary placentæ in Hyrax and Elephas does not necessarily afford any proof of affinity of these types with the Carnivora. A zonary placenta may quite easily be derived from a diffused placenta; and the presence of two villous patches at the poles of the chorion in Elephas indicates that this was very probably the case with the placenta of this form. Although it is clear from the above considerations that the placenta is capable of being used to some extent in classification, yet at the same time the striking resemblances which can exist between such essentially different forms of placenta, as for instance those of Man and the Rodentia, are likely to prevent it being employed, except in conjunction with other characters. _Special types of development._ The Guinea-pig, Cavia cobaya. Many years ago Bischoff (No. 176) shewed that the development of the guinea-pig was strikingly different from that of other Mammalia. His statements, which were at first received with some doubt, have been in the main fully confirmed by Hensen (No. 182) and Schäfer (No. 190), but we are still as far as ever from explaining the mystery of the phenomenon.