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 



coel 



FIG. 1235. Diagram of the embryo and 

 foetal membranes of Hypsiprymnus 

 rufescens. all. allantoic cavity ; amn. 

 amnion ; amn. c. cavity of amnion ; coel. 

 extra-embryonic coslome ; ser. serous 

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

 (After Semon.) 



all 



FIG. 1236. Diagram of the embryo 

 and total membranes of Phas- 

 colarctos cinereus. Letters 

 as in Fi<r. 12,55. (After Semon.) 



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 two^'eggs which it pro- 

 duces are deposited in its 



FIG. 12:57. -:Digrinofthee^andpta*entaQf burrow . In Echidna the egg- 

 shell is composed of keratin ; 

 in Ornithorhynchus it con- 

 tains 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 merobUstic, an4 the blastoderm 



Perameles obesula. 



In addition, all. s. allantoic stalk ; mes. mesen- 

 chyme of outer surface of allantois fused with 

 mesenchyme of serous membrane ; s. t. sinus 

 terminalis ; ut. uterine wall. (After J. P. Hill.) 



