XIII 



PHYLUM CHORDATA 



563 



which is enabled to breathe unobstructedly through the nostrils by 

 the establishment of a continuous passage from the nasal cavities 

 to the larynx, as already described (p. 544). 



In all the Marsupials, so far as known, the embryo is covered 

 over, except in a limited area, by the compressed and expanded 



amn 



coei 



Fio. 1154. Diagram of the embryo and foetal mem- 

 branes of Hypsiprymnus rufescens. //. 

 allantoic cavity; nmn. amnioii ; amn. c. cavity 

 of amnion ; c&7. extra-embryonic coelonis ; m-r. 

 serous membrane ; ?/.. yolk-sac. (After ISemon.) 



all 



Fio. 1155. Diagram of the embryo and 

 ftetal membranes of Phascolarctos 

 cinereus. Letters as in Fig. 1154. 

 (After Semoii.) 



coel 



arrwi 



yolk-sac. In the majority (Fig. 1154) the allantois (all.) is small 

 and is completely enclosed with the embryo in the yolk-sac. In 

 the Koala, however (Fig. 1155), it stands out and becomes closely 

 applied to the serous mem- 

 brane over the small area 

 not covered by the yolk- 

 sac ; but no vascular villi 

 are developed. Only in the 

 Bandicoots (Fig. 1156), so 

 far as known, is the out- 

 growth of the allantois to 

 the serous membrane fol- 

 lowed by the establishment 

 of a true placental con- 

 nection by the development 

 of vascular villi received 



all 

 mes 



all.s 



si 



S.US 



FIG. 1156. Diagram of the embryo and placenta of 

 Perameles obesula. Letters as in Fig. 1154. 

 In -addition, at 1. s. allantoic stalk; mes. niesen- 

 chyme of outer surface of allantois fused with 

 meseiichyme of serous membrane ; .?. t. sinus 

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



into uterine crypts. 



The Prototheria, unlike 

 all the rest of the Mam- 

 malia, 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. 464) in the mammary region of the ventral 

 surface. The young animal soon emerges from the egg, and 



o o 2 



