AMNION AND ALLANTOIS. 



741 



Fig. 256. 



Diagram of the FECUN- 

 DATED EGG, farther 

 advanced a. Umbilical 

 vesicle, b. Amniotic cav- 

 ity, c. Allantois. 



Fig. 257. 



rising up of the amniotic fold. This diverticulum is the commencement 

 of the allantois. It is an elongated membranous sac, continuous with 

 the posterior portion of the intestine, and con- 

 taining bloodvessels derived from those of the 

 intestinal circulation. The cavity of the allantois 

 is also continuous with the cavity of the intes- 

 tine. 



After the amniotic folds have approached and 

 touched each other, as above described, over the 

 back of the embryo, the adjacent surfaces, thus 

 brought in contact, fuse together, so that the 

 cavities of the two folds, coming respectively from 

 front and rear, are separated only by a single 

 membranous partition (Fig. 251, c) running from 

 the inner to the outer lamina of the amniotic 

 folds. This partition is soon afterward atro- 

 phied and disappears ; and the inner and outer laminae become conse- 

 quently separated from each other. The inner lamina (Fig. 257, a) 

 which remains continuous with the integument of the foetus, inclosing 

 the body of the embryo in a distinct cavity, is 

 called the amnion (Fig. 258, 6), and its cavity is 

 known as the amniotic cavity. The outer lamina 

 of the amniotic fold, on the other hand (Fig. 

 257, 6), recedes farther and farther outward, 

 until it comes in contact with the original vitel- 

 line membrane, still covering the exterior of the 

 egg. It at last fuses with the vitelline membrane 

 and unites with its substance, so that the two 

 form but one. This membrane, resulting from 

 the union and consolidation of two others, con- 

 stitutes then the external investing membrane of 

 the egg. 



The allantois, in the mean time, increases in 

 size and vascularity. Following the course of 

 the amniotic folds as before, it insinuates itself 

 between them, and thus comes in contact with 

 the external membrane above described. It then 

 begins to expand laterally, enveloping more and 



more the body of the embryo, and bringing its vessels into contact with 

 the external investing membrane of the egg. 



By a continuation of this process, the allantois at last envelops com- 

 pletely the body of the embryo, together with the amnion; its two 

 extremities coming in contact with each other, and fusing together over 

 the back of the embryo, in the same manner as the amniotic folds had 

 previously done. (Fig. 258.) It lines, therefore, the whole internal 

 surface of the investing membrane with a flattened, vascular sac, the 



Diagram of the FECTTN. 

 DATED EGG, with allan- 

 tois nearly complete. a. 

 Inner lamina of amniotic 

 fold. 6. Outer lamina of 

 ditto, c. Point where the 

 amniotic folds come in 

 contact. The allantois is 

 seen penetrating between 

 the inner and outer lami- 

 nae of the amuiotic folds. 



