XIII 



PHYLUM CHORDATA 



FIG. 1225. Diagram of the embryo and placenta of 

 Perameles obesula. Letters as in Fig. 1154. 

 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.) 



closely applied to the chorion over the small area not covered 

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

 Native Cat (Dasyurus) there 

 is a well-developed yolk- 

 sac placenta. Only in the 

 Bandicoots (Fig. 1225), 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, unlike 

 all the rest of the Mammalia, are oviparous. 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. 

 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 

 S 12-20 -.<, biastuia stage of one of the O f a large proportion of food yolk. 



Theria. B, transition stage between _.. r . - J _ 



the momia and biastuia in a Mono- The segmentation, unlike that of 



^1 the Theria, is meroblastic, 



