596 



ZOOLOGY 



of the Eutheria not only in the non-occurrence, as already 

 mentioned, of the solid morula-like stage, but also in the fact 

 that the layer corresponding to the embryonal cell-mass is never 

 enclosed by the trophoblast — Eauber's layer being absent ; and in 

 the transitory characterj or entire absence, 

 of an allantoic placental connection be- 

 tween the foetus and the uterine mucous 

 membrane. The intra-uterine develop- 

 ment of the foetus is abbreviated, and 

 birth takes place when the young animal 

 is still relatively very small and has many 

 of the parts incompletely formed. In this 

 helpless condition the young Marsupial 

 is placed by the mother in the marsupium, 

 where it remains for a time as a mam- 

 mary fwtus (Fig. 1222), hanging passively 

 to the teat, to which the mouth becomes 

 firmly adherent. The milk is expressed 

 from the mammary gland by the con- 

 traction of a muscle, the cremaster, and 

 passes down the gullet of the foetus, 

 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. 576). 



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

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



Fig. 1222.— Mammary fcetus of 

 Kangaroo attached to the 

 teat. (Natural size.) 



coel-^. 



coeL 



Fig. 1223 — Diagram of tlie embryo and fcetal mem- 

 V)ranesof Hypsiprymnusrufescens. all. 

 allantoic cavity ; amn. amnion ; wtnn. c. cavity 

 of amnion ; ci£l. extra-embryonic coelpme ; ser. 

 serous membrane; yle.n. yolk-sac. (After Semon.) 



Fig. 1224. — Diagram of the embryo and 

 foetal membranes of Fhascolarctos. 

 cinereus. Letters as in Fig. 1164, 

 (After Semon.) 



yolk-sac. In the majority (Fig. 1223) the allantois (aZZ.) is small, 

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

 In the Koala, however (Fig. 1224), it stands out and - becomes 



