702 



MAMMALIA. 



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. Development 

 begins in the oviducts, but the eggs 

 are in no way attached to the wall. 

 They are laid in a nest by the Duck- 

 mole ; in the Echidna they are hatched 

 in a slight, periodically developed, ex- 

 ternal pouch. 



(b) In the Marsupials the connec- 

 tion between mother and offspring has 

 become closer. The embryo is born 

 alive, but prematurely and after a short 

 gestation. Till recently it was believed 

 _ that during its intra-uterine life it was 



FIG. 345. Development either not attached to the wall of the 

 of hedgehog. Three uterus at all, or only to a slight extent 

 H^brecht geS '" After b y a yolk-sac placenta. It is now 

 i. Shows internal vesicle of known however, that, in Peraineles at 

 hypobiast ; the disc and least, there is not only an efficient 

 bfafr^t^ow? vSii yolk-sac placenta, but a distinct, though 

 arising from trophobiast^ small, allantoic placenta. The general 



the disc of formative epi- , r , -*- i 



blast (/#.); the biasto- absence of a placenta in Marsupials, 



vesicle (Z-'.r.). _ n J j-U p cm-ill n\7t* nf thp 

 ai 



dermic 



III. A more advanced 



stage; 7>., trophobiast ; must therefore be ascribed to degenera- 

 Sastf^^bSdernSic tion, and not to a primitive condition. 

 vesicle; //., hypobiast. The presence of a yolk-sac placenta in 



Marsupials is not in itself of great 

 importance, for a connection between the yolk-sac of the 

 embryo and the wall of the oviduct exists in two Elasmo- 



