DEVELOPMENT AND PLACENTATION. 



649 



I 



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 de- 

 veloped, external pouch. 



ip) In the Marsupials, 

 the connection between 

 mother and offspring has 

 become closer. The em- 

 bryo 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 opos- 

 sum, it lies surrounded by 

 a quantity of nutritive al- 

 buminoid material. Here 

 it may be recalled that in 

 two Elasmobranch fishes 

 and in two lizards, there 

 is a connection between 

 the yolk sac of the embryo 

 and the wall of the ovi- 

 duct. As we shall see, 

 there is a preliminary yolk 

 sac placenta in three orders 

 of Placentals. 



{c) In all the other Mam- 

 mals, the maternal sacrifice 

 prior to birth is greater, for a close connection is established 

 between the embryo and the wall of the uterus, by means 



Fig. 225. — Development of 

 Hedgehog. Three early stages. 

 (After HuBRECHT.) 



I. Shows internal vesicle of hypoblast, 

 the disc and external sheath of epiblast. II. 

 Shows villi arising from trophoblast ; the 

 disc of formative epiblast {Kfi.)\ the blasto- 

 dermic vesicle (/>'.7'.) III. A more ad- 

 vanced stage, Tr., trophoblast : Ep.^ disc 

 of formative epiblast; Z>.7'. , blastodermic 

 vesicle ; H., hypoblast. 



