DEVELOPMENT AND PLACENTATION. 



649 



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. 

 u. Y/ I They are laid in a nest 



by the Duckmole; in the 



y *^J5fe'-r P L Echidna they are hatched 



jf \ in a slight, periodically de- 



/' veloped, external pouch. 



I 2 (b) In the Marsupials, 



Jr 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 



B.v 





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 (Ep.} ; the blasto- 

 dermic vesicle (B.v.) III. A more ad- 

 vanced stage, TV., trophoblast ; Ep., di.sc 

 of formative epiblast ; B.v., blastodermic 

 vesicle ; H., hypoblast. 



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

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



