578 



ZOOLOGY 



SECT. 



Only in the Bandicoots (Fig. 1237), so far as known, is the out- 

 growth of the allantois to the chorion followed by the establishment 

 of an intimate relationship between the chorion and the uterine 

 wall, with the formation of interlocking ridges and depressions, the 



r/mn 



cotl 





all 



FIG. 12:35. Diagram of the embryo and 

 foetal membranes of Hypsiprymnus 

 rufcscens. nil. allantoic cavity ; amn. 

 ainnion ; amn. c. cavity of amnion ; cael. 

 extra-embryonic coelome ; ser. serous 

 membrane (chorion); yk.s. yolk-sac. 

 (After Semon.) 



FIG. 1236. Diagram of the embryo 

 and foetal membranes of Phas- 

 colarctos cinereus. Letters 

 as in Fi'i. 12 !5. (After Semon.) 



cod 



all 



afl.s 



whole constituting a placenta of the same essential character as' 

 that of the Eutheria, though devoid of actual villi. 



The Prototheria, unlike all the rest of the Mammalia, are ovi- 

 parous. In Echidna only a single egg, as a general rule, is laid in 



a season. This is placed in 

 a temporary marsupium, 

 formed as already described 

 (p. 477) in the mammary 

 region of the ventral surface. 

 The young animal soon 

 emerges from the egg, and 

 remains enclosed in the mar- 

 supium till it reaches an 

 advanced stage of develop- 

 ment. Ornithorhynchus de- 

 velops no marsupium, and 

 the two7eggs which it pro- 

 duces are deposited in its 



FIG. 1237. Diagram of the embryo and placenta of k mTOW T n Echidna the 622- 

 Perameles obesula. Letters a* in Fig. 1235. u f 



In addition, all. s. alUintoie stalk ; rues, mesen- shell is Composed OI keratin J 



chyme of outer surface of allantois fused with . _ . , r , -, 



mesenchyme of serous membrane ; *. t. sinus in UmithomyncnUS It COn- 



terminaiis ; . uterine wa i!. ^^ carbonate of lime. The 



ova of the Prototheria, (Fig. 1238) are very much larger than 

 those of other Mammals, their greater dimensions being due to 

 the presence of a large proportion of food-yolk. The segmentation, 

 unlike that of all the Theria, is meroblastic ? and the blastoderm 



sf 



