At a meeting 1 of the Central Veterinary Medical Society of London, at which I exhibited a number of young horse embryos, I had barely time to allude to what may prove a subject of some practical importance, viz., the apparatus provided for nourishing the embryos during the early weeks of development. This apparatus (which consists mainly of what are known as the foetal appendages) I was led to consider from a breeder's point of view by Lord Arthur Cecil asking me, about a year ago, if I could account for so many mares breaking service from the sixth to the ninth week. At the time I could only answer that I agreed with his lordship in believing it was in many cases due to the detachment and escape of an embryo. I have frequently found mares all right, as far as I could judge, at the end of the sixth week, and all wrong some weeks later. This seems to be not an uncommon experience, more especially at the beginning of the breeding season, hence it is always urged by experts that mares should be " tried " frequently during at least the first two months. If holding at the end of the ninth week no further trouble is, as a rule, anticipated. But no one has, as far as I am aware, explained why the breeder experiences so many difficulties during the earlier weeks. According to the evidence obtained by the Royal Commission on Horse Breeding, it appears that about 40 per cent, of the mares selected for breeding fail to produce offspring during any given year. This is a very high percentage 1-1th March (see the Field, 13th March 1897). b A CRITICAL PERIOD IN THE of failure, but from reports recently received it seems to be still higher in certain districts in India. 1 Believing that some of my mares have proved unfruitful for the time being because of their frequently breaking service, and believing further that not a few of the disappointments that fall to the lot of breeders result from the same cause, I decided to examine material collected for another purpose, with a view to shedding fresh light on this difficult but important practical question. To admit of the conditions which obtain in the brood mare being more easily understood, I shall at the outset refer to the apparatus provided for the nourishment of the chick, and to the corresponding structures in one of the simpler mammals. The Chick's Foetal Appendages. 2 Every one who has examined a hen's egg at, say, the end of the ninth day of incubation, knows that the already well-formed chick is, as it were, mounted on a large, well-filled forage bag the yolk sac-from which it hour by hour draws its nourishment. But in addition to the familiar yolk sac there is a second sac, in its way quite as important, though empty of food, viz., the thin compressed sac lying in contact with the shell, which plays the part of a breathing organ or lung. This second sac is known as the allantois.