THYLUlNr CHORDATA 



507 



closely applied to tlie chorion over the small area not covered 

 by the yolk-sac ; but no vascular villi are developed. In tlie 

 Native Cat {Da-^j/unis) there 

 is a well-develoj^ed yolk- 

 sac placenta. Only in the 

 Bandicoots (Fig. 122.5), so 

 far as known, is the out- 

 growth of the allantois to 

 the chorion followed by 

 the establishment of an in- 

 timate relationship between 

 the latter and the uterine 

 wall, with the formation of 

 interlocking ridges and de- 

 pressions, the whole con- 

 stituting a placenta of the 

 same essential character 

 as that of the Eutheria, 

 though devoid of actual 

 villi. 



The Prototheria, 

 all the rest of the 

 a single egg, as a 



k;. 122.',, — r>ia^a-,iiii of tlie embryo and placenta of 

 Perameles obesula. Letters as in Pig. IVA. 

 In adjiti'in — nll.^. allantoie stalk; m<>.'i. mesen- 

 chyme of onter surface of allantois fused with 

 mesenchyme of serous mernltrane ; ?. 1. sinus 

 terminalis ; I't. uterine wall. (After .J. P. Hill.) 



Fic. 122i;.— ,J, Ijlastiila stage of one of the 

 Theria. B, transition stage between 

 tlie morula and blastula in a Mono- 

 treme. Both represented in diagram- 

 inatie section. (After Semon.) 



unlike 



Mammalia, are oviparous. In Echidna onl}^ 

 general rule, is laid in a season. This is 

 placed in a temporary marsupium, 

 formed as already described (p. 

 491) in the mammary region of 

 the ventral surface. The young- 

 animal soon emerges from the 

 egg, and remains enclosed in 

 the marsupium till it reaches 

 an advanced stage of develop- 

 ment. Ornithorhynchus develops 

 no marsupium, and the two 

 eggs which it produces are 

 deposited in its burrow. In 

 Echidna the egg-shell is com- 

 posed of keratin ; in Ornitho- 

 rhynchus it contains carbonate 

 of lime. The ova of the Pro- 

 totheria (Fig. 1226) are very 

 much larger than those of other 

 Mammals, their greater dimen- 

 sions being due to the presence 

 of a large proportion of food yolk. 

 The segmentation, unlike that of 

 all the Theria, is meroblastic. 



