8 A CRITICAL PERIOD IN THE 



amnion {am.), has suspended from it a large yolk (y.s.) sac and a 

 relatively small allantois (all.). Surrounding the embryo and its 

 appendages is an outer tunic, which may be known as the outer 

 embryonic sac (a, h, c, d). With part of this embryonic sac the 

 yolk sac blends (from a to c), and it is through this area (a, b, c) 

 the nourishment — the uterine milk — enters, or is absorbed into, 

 the yolk sac. The nutriment is at once conveyed to the embryo 

 by the blood-vessels, which plentifully ramify in that part of the 

 sac which has not fused with the outer tunic. In other words, 

 through the bloodless area (a, b, c) nourishment diffuses from the 

 uterus into the yolk sac, thence into the yolk sac vessels, which 

 carry it to the embryo. The embryonic sac does not simply lie 

 in the uterus, it is fixed or cemented, and retained in one definite 

 position. This fixing is mainly secured by cells of the outer 

 tunic in the absorbing area adhering to cells of the uterine lining 

 or mucous membrane. In the opossum and kangaroo and the 

 vast majority of the marsupials these adhesions, at the best very 

 feeble, are in a few weeks broken down, the embryonic sac 

 ruptures, and the embryo, still in a very immature and helpless 

 condition, escapes, and is eventually suspended to a teat, which 

 may or may not project into a pouch or marsupium. It is 

 especially worthy of note that in the opossum the allantois takes 

 no part in securing nourishment from, or in fixing the embryonic 

 sac to, the uterine wall. 



The Fcetal Appendages in the Horse. 



While opossums and kangaroos and many other ancient and 

 primitive creatures appear on the scene in an immature and 

 helpless condition, some of the higher mammals, more especially 

 some of the members of the hoofed or ungulate tribe, are, as 

 every one knows, at birth so perfect in all their parts that they 

 can at once join and keep pace with the herd to which they by 

 birth belong. Mr Hudson, in speaking of the sheep on the 

 Pampas, says: — " I have often seen a lamb dropped on the frosty 

 ground in bitterly cold windy weather in midwinter, and in less 

 than five seconds struggle to its feet, and seem as vigorous as any 

 day-old lamb of other breeds. The dam, impatient at the short 



