XIII 



PHYLUM CHORDATA 



507 



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 (Dasi/iirus) 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 

 interlockino- rids^es and de- 

 pressions, the whole con- 

 stituting a placenta of the 

 same essential character 

 as that of the Eutheria, 

 thou oh devoid of actual 



Fii:. l'2-2."i.— |)ia<,'r:uii of the emliiyo and placenta of 

 Ferameles obesula. Luttci-s as in Fig. 1154. 



In aiMiti(in — ii//. .s'. allantoic stalk; mi'o. meseii- 

 ohyme of outer surface of allantois fused with 

 inescnchynie of serous mcmlirane ; .s-. t. sinus 

 terminalis ; «f. uterine wall. (After J. P. Hill.) 



villi. 



The Prototheria, unlike 



all the rest of the Mammalia, are oviparous. In Echidna only 



a single ego- as a general rule, is laid in a season. Tins 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 



Fio. i2-2ij.—A, biastuia stage of one of the of a large proportion of food yolk. 



Tberia. B, transition stage between ^ni x \l- ti xl i. r 



thu morula and biastuia in a Mono- 1 hc Segmentation, UnllKC tJiat 01 



treme. Both represented in diagram- ^11 tliP^Thprin 1=; mpvoblastic 

 jnatic section. (After Semoji. ) ^^^ "^"6 iUeua, IS lUeiOUiabUlC. 



