DEVELOPMENT OF THE HOUSE. 11 



mature escape is all but impossible. Should one of these 

 embryos succumb in the struggle for life (which often begins 

 before birth), it still remains in its pouch, and is bit by bit 

 absorbed. From first to last in the case of the mare the con- 

 nections between the embryonic sac and the uterus are, compared 

 even with other ungulates, easily broken down. As I have on 

 several occasions observed, the moment an incision is carried 

 upwards from the neck into the horn of the still contractile 

 uterus, the embryonic sac begins to protrude, and, still entire, 

 the sac may be at once forced, by the contractions of the uncut 

 upper part of the horn, into the body of the uterus. Hence, if 

 the neck of the double horned uterine sac is in a flaccid and 

 unhealthy condition, or if strong wave-like contractions are set up 

 in the uterine horns, there is little chance of a horse embryo only 

 a few weeks old long retaining its position, even if well cemented 

 to the uterine mucous membrane. The need of supplementary 

 anchorages is early recognised in the case of the horse. Even 

 at the fourth week a ring (an., fig. 3), consisting of delicate 

 nearly parallel ridges, exists around the absorbing area. These 

 ridges not only dip into depressions in the lining of the uterus, 

 but in addition many of the cells covering them blend with the 

 adjacent uterine cells. Further, some of the cells forming the 

 outer tunic or embryonic sac increase in length, so as to form an 

 indistinct belt or girdle, nearly on a level with the embryo. This 

 girdle occupies in a section the position indicated by the letters 

 t.g., in fig. 3. 



The Five- Weeks' Horse Embryo. — If we turn from the four to the 

 five-weeks' embryo we find various important changes have taken 

 place. The embryo, now five-eighths of an inch in length, has all 

 but lost its gill slits, while a rudiment of the ear has appeared in 

 connection with the remnant of the first cleft ; the tail is nearly 

 straight, and, compared with the limbs, relatively shorter than at 

 four weeks, and the limbs seem to contain at their tips rudiments 

 of three separate digits ; at four weeks the limbs are quite un- 

 differentiated. In fig. 4, which represents the actual size of the 

 various structures, it will be noticed (1) that while the yolk sac 

 (y.s.) is larger, the absorbing area (a, b, c) is smaller than at four- 

 weeks ; (2) that the capacity of the allantois (all.) has greatly in- 

 creased ; and (3) that the girdle (t.g.) is more pronounced and 



