596 



ZOOLOGY 



SKCT. 



of tlie Eutlieria 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 — Rauber's layer being absent ; and in 



the transitory character, or entire absence, 

 of an allantoic placental connection be- 

 tween the foetus and the uterine mucous 

 niembr.ine. The intra-uterine develop- 

 ment of the fuetus 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 fcehts (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 gidlet 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. 570). 



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

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



Fid. 12J-2.— Mumiiiarv fietiis of 

 Kangaroo attaciiod tn the 

 teat. (Natural .size.) 



amn 



Fk;. 1223 — Diagi-am of the embryo and fietalmem- 

 1)raiie.s of Hypsiprymnus rufescens. all. 



allantoic cavity ; iit:i,i. aiiiiiic.n ; nu.ii.c. cavitj' 

 of amnion ; ixd. e.xtra-embryouie cielome ; ncr. 

 serous membrane; yk.s. yolk-sac. (After Semon.) 



coel 



Fio. 1 2-24.— Diagram of (he embryo and 

 fntal nicmbranes of FhascolarctOS 

 cinereus. Letters ;is in l-if<, li:.4. 

 (After Seuion.) 



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

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

 In the Koala, however (Fig. 122-1), it stands out and. becomes 



