10 A CRITICAL PERIOD IN THE 



The Four- Weeks' Horse Embryo. — Fig. 3 represents in a semi- 

 cliagrammatic way, but as near the actual size as possible, a four- 

 weeks' horse embryo and its various appendages and tunics. The 

 embryo, which was found coiled with the tail overlapping the 

 head region, measured just under three-eighths of an inch in 

 length. This embryo, it will be observed, like the chick and 

 opossum, is closely invested by an inner tunic, the amnion {am.), 

 and connected by a short stalk with a large yolk sac {y.s.). It is 

 also connected by a second stalk (not shown in the figure) with 

 a still relatively small allantois {all.). Surrounding and protecting 

 the embryo and its special appendages, is the thin outer tunic 

 {a, b, c, d). In the wall of the allantois there are many 

 blood-vessels, and already this sac, by bringing the foetal flood 

 sufficiently near the blood coursing through the uterus, may 

 admit of an exchange of gases taking place — it may, in fact, 

 already play the part of a breathing organ, the oxygen being 

 derived from the maternal blood in very much the same way as 

 in the case of the fish it is derived from the water. As in the 

 opossum, part of the yolk sac (from a to c) has blended with the 

 outer tunic. Through this bloodless area nutritive material 

 passes into the yolkless sac. With the exception of this bloodless 

 part of the yolk sac, its walls are crowded with blood-vessels. 

 One large vessel which carries blood from the embryo forms a 

 complete circle around the absorbing area ; from this circular 

 vessel numerous branches radiate upwards, forming on the way 

 an elaborate network. The blood is eventually collected from 

 this network into two large vessels (veins), which convey it back 

 again to the embryo. As the blood courses through the wall of 

 the yolk sac, some of the nutriment which has entered through the 

 absorbing area {a, I, c) is added to it. The yolk sac is thus a 

 reservoir into one end of which nutritive material filters from 

 the uterus, and then diffuses into the vessels of the sac, in 

 order to find its way to and supply the ever-increasing wants of 

 the embryo. As in the opossum, the outer tunic is anchored to 

 the lining of the uterus by some of the cells occupying the absorb- 

 ing area {a, b, c) ; but, as in the opossum, the connections are 

 easily broken down. This is in striking contrast with what 

 occurs in certain other mammals. In some cases a special uterine 

 pouch is prepared beforehand for each embryo, from which pre- 



