A Critical Period

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Excerpt from A Critical Period: In the Development of the Horse 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. 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. The position and relations of the yolk sac and allantois are indicated in fig. 1. Through the walls of both sacs blood is constantly passing to and from the chick, collecting particles of altered yolk from the one and fresh supplies of oxygen (direct from the air which penetrates the shell) by means of the other. About the Publisher Forgotten Books publishes hundreds of thousands of rare and classic books. Find more at www.forgottenbooks.com This book is a reproduction of an important historical work. Forgotten Books uses state-of-the-art technology to digitally reconstruct the work, preserving the original format whilst repairing imperfections present in the aged copy. In rare cases, an imperfection in the original, such as a blemish or missing page, may be replicated in our edition. We do, however, repair the vast majority of imperfections successfully, any imperfections that remain are intentionally left to preserve the state of such historical works.