540 MAMMALS. 



The Connection between Embryo and Mother. — (a) The 

 lowest Mammals, the Duckmole \Ornithorhynchus) and the 

 Porcupine Ant-Eater {Echidna) resemble Birds and most 

 Reptiles in bringing forth their young as eggs, i.e., in 

 being oviparous. The eggs are large, with a considerable 

 quantity of yolk, and after fertilisation divide partially, i.e., 

 exhibit meroblastic segmentation like the eggs of Birds 

 and Reptiles. The tunic formed round about them in the 

 Graafian follicles of the ovary consists as in Birds and 

 Reptiles of a single layer of cells. As they develop they are 

 unattached to the walls of the oviducts. They are laid in a 

 nest by the Duckmole ; in the Echidna they are hatched in 

 a slight, periodically developed, external pouch. 



{U) In the Marsupials, the connection between mother 

 and offspring has become closer. The embryo is born alive, 

 though prematurely, after a short uterine life, during which, 

 however, it is either not vitally attached at all to the uterus, 

 or only to a slight degree by villi from the yolk-sac. In 

 the opossum, it lies surrounded by a quantity of nutritive 

 albuminoid material. Here it may be recalled that in two 

 Elasmobranch fishes and in two lizards, there is a connec- 

 tion between the yolk-sac of the embryo and the wall of 

 the oviduct. 



(c) In all the other Mammals, the maternal sacrifice prior to 

 birth is greater, for a close connection is established between 

 the embryo and the wall of the u,terus, by means of a special 

 adaptation — the placenta. This, in rough physiological lan- 

 guage, is a double vascular sponge, partly embryonic, partly 

 maternal, by means of which the blood of the mother 

 nourishes and purifies that of the embryo. 



As many of the most fundamental structural and func- 

 tional problems in connection with placentation are still 

 being investigated, it is impossible to discuss even the lead- 

 ing questions with definiteness and certainty. The authority 

 here followed is Hubrecht, who has recently investigated in 

 great detail the placentation of the hedgehog, which is at 

 once a simple and a central type. 



First, then, let us seek to define the embryonic and 

 maternal structures which are associated with placentation. 

 (i) At a very early stage, the divided ovum of the hedgehog 

 consists of a sac of outer-layer, epiblastic or ectodermic 



