General and Professional Biology; With Special Reference to Man (Paperback)

This historic book may have numerous typos and missing text. Purchasers can download a free scanned copy of the original book (without typos) from the publisher. Not indexed. Not illustrated. 1922 edition. Excerpt: ...cover the embryo. The inner one is the ectodermal layer just mentioned and the outer one is known as the serosa. There is a sero-amniotic cavity between the two. The somatopleure now extends peripherally until the entire yolksac, and eventually the embryo as well, is covered with serosa. The allantois extends between serosa and amnion. THE ALLANTOIS This structure differs from the amnion and yolk-sac in that it develops within the embryo proper, though it does extend out into the extra-embryonic region as it develops. About the third day, the allantois develops by an outpushing of the ventral wall of the hind-gut (entoderm), pushing the splanchnopleure ahead of it, so that we may say it is composed of splanchnopleure with an entodermal lining. The following day it pushes out of the embryo into the extra-embryonic coelom, the attached end lying caudal to, and parallel to, the yolk-stalk. The proximal portion is called the allantoic stalk, and the extended bladder-like distal portion the allantoic vesicle. It grows very rapidly from the fourth to the tenth day, and extends into the sero-amniotic cavity in a flattened manner. Ultimately it encompasses the entire embryo and yolk-sac, and in so doing the mesodermic layer of the allantois fuses with the layer of mesoderm of the serosa which comes to lie in direct contact with it. This means that there is thus formed a double layer of mesoderm, the serosal portion being derived from the somatic mesoderm, and the allantoic portion derived from the splanchnic mesoderm. A very rich vascular network now develops between these two layers connected with the vascular circulation by the allantoic arteries and veins. The allantois thus becomes an organ of respiration, as well as circulation, to the developing...