xui PHYLUM CHORDATA 577 



In many Mammals the yolk-sac, through the medium of the 

 chorion, enters into a close relationship with the uterine wall, and 

 a connection, the so-called yolk-sac placenta, is established through 

 which nourishment can be conveyed to the embryo ; but this rarely 

 persists after the true (allantoic) placenta has become established. 



The stalk of the yolk-sac, with the corresponding narrowed part 

 of the allantois and the vessels which it contains, forms the umbilical 

 cord by which the foetus is connected at the umbilicus with the 

 yolk-sac and placenta. This is enclosed in a sheath formed by the 

 ventral portion of the amnion. The part of the allantois which 

 remains within the cavity of the body develops into the urinary 

 bladder, together with a cord the urachus connecting the bladder 

 with the umbilicus. 



In the Marsupials the ovum is comparatively large. After 

 fertilisation it becomes enclosed in a thick shell-membrane with a 

 layer of albumen. The first cleavage of the 

 ovum (Didelphys), or the fourth (Dasyurus), 

 involves a separation of the embryo-forming 

 part of its substance from that destined to 

 give rise only to the trophoblastic ectoderm. 

 Further divisions take place in such a way as 

 to give rise, not to a solid morula as in the 

 Eutheria, but to a hollow blastodermic vesicle 

 with a wall composed of a single layer, the 

 cells on one side of which form the embryonic 

 area. An allantoic placenta is not developed 

 except in Perameles. The intra-uterine deve- 

 lopment of the foetus is abbreviated, and birth 

 takes place when the young animal is still Flo . i 2 34.-Mammary foetus 

 relatively very small and has many of the * ^ a ^ ga /^? c i att f c ! ie( ! to 



. J /.I e i T , i i i i the teat. (Natural size). 



parts incompletely formed. In this helpless 



condition the young Marsupial is placed by the mother in the mar- 

 supium, where it remains for a time as a mammary foetus (Fig. 1234), 

 hanging passively to the teat, to which the mouth becomes firmly 

 adherent. The milk is expressed from the mammary gland by the 

 contraction 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. 558). 



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

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

 yolk-sac. In the majority (Fig. 1235) the allantois (all.) is small, 

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

 the Koala, however (Fig. 1236), it stands out and becomes closely 

 applied to the serous membrane 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. 



