GENERATION AND DEVELOPMENT 715 



and the blastocyst (that is, the external cover of the cellular mass) 

 lies in contact with the uterine epithelium. On the twelfth day the 

 ovum has reached nearly to the lower limit of the horn in which it 

 lies, the glands of the uterus enlarge, and the blastocyst rapidly 

 elongates so that each end grows out to the tip of each horn of the 

 uterus. If one embryo only be present, it extends through both 

 horns of the uterus ; if there are two, they are each confined to one 

 horn. On the seventeenth and eighteenth day the first attachment 

 of the embryo to the uterus is effected, a very important period 

 in embryonic life. Up to this time the only nourishment available, 

 exclusive of the yolk sac, is that furnished by the juices poured into 

 the uterine cavity by the glands, and until the twentieth day the 

 ovum receives no other source of supply but this. On the twenty- 

 eighth day villi on the external covering of the embryo are well 

 developed, and on the maternal cotyledons are little depressions, 

 into which they fit. The allantois has grown rapidly, and the yolk sac 

 has become reduced as the allantois increases. By the forty-fourth 

 day the fcetal cotyledons are scattered over the whole surface of 

 the embryonic covering. On the seventy-eighth day the general 

 character of the placenta is established. As the uterus swells, 

 owing to the increase in size of its contents, it does so generally 

 excepting the upper part of the horns, which are but little longer 

 than normal, and are engaged in active secretion. 



This condensed account from Assheton of the development 

 of the embryo of the sheep is the first exact knowledge of how 

 the embryo of a ruminant comports itself during the early days 

 of development. 



The implantation of the embryo of the horse has been dealt 

 with by Ewart,* not with the same degree of fulness as the 

 above, as that is practically impossible, but sufficiently so to 

 show not only the characteristic features of the process, but 

 their profound practical bearing on the hygienic care of brood 

 mares. 



The human decidua grows over the ovum on its arrival in the uterus, 

 and so prevents its escape (see p. 708) . No such pouch is formed in the 

 ungulates, and the escape of the ovum before it is securely fixed to 

 the wall of the uterus is not unlikely, especially in the horse, where 

 the connection between the embryonic sac and the uterus is easily 

 broken down. To understand how this occurs, Ewart points out 

 that the remote ancestors of the horse were probably born on the 

 forty-seventh or forty-eighth day of conception, and, like the 

 ancient and primitive mammals, the opossum and kangaroo, passed 

 from the uterus to a pouch, where they lay securely suspended by 

 a teat until their perfect development was completed. The arrange- 

 ment by which the equine embryo is anchored, as Ewart calls it, 

 to the wall of the uterus is in the first instance by some of the cells 

 of the outer layer of the embryo, at a part which is in communica- 

 tion with the yolk sac. This connection (Fig. 251, a, b, c) is of a 

 very slender kind, and is the only one which exists up to the fifth 



* ' A Critical Period in the Development of the Horse,' by T. C. Ewart, 

 M.D., F.R.S., 1897. 



