600 AMNION AND ALLANTOIS. 



consequently it can nowhere come in contact with the investing 

 membrane of the egg. But in the higher animals, the internal 

 blastodermic layer, which is the seat of the greatest vascularity> 

 and which is destined to produce the allantois, is made to come in 

 contact with the external membrane of the egg for purposes of 

 exhalation and absorption ; and this can only be accomplished by 

 opening a passage for it through the external germinative layer. 

 This is done in the following manner, by the formation of the 

 amnion. 



Soon after the body of the foetus has begun to be formed by the 

 thickening of the external layer of the blastodermic membrane, 

 a double fold of this external layer rises up on all sides about 

 the edges of the newly-formed embryo ; so that the body of the 

 foetus appears as if sunk in a kind of depression, and surrounded 

 with a membranous ridge or embankment, as in 

 Fig. 207. Fig. 207. The embryo (c) is here seen in profile, 



with the double membranous folds, above men- 

 tioned, rising up just in advance of the head, 

 and behind the posterior extremity. It must be 

 understood, of course, that the same thing takes 

 place on the two sides of the foetus, by the forma- 

 of FKCU*- tion of lateral folds simultaneously with the 

 kot- appearance of those in front and behind. As it 

 a. vueiius. b. External i s these folds which are destined to form the 



layer of blastodermic . .-, 111^1 n . r i i 



membrane, c. Body of amnion, they are called the " ammotic folds, 

 embryo, d, d. Amniotic The amniotic folds continue to grow, and ex- 



folds. e. Vitelline mem- ,.. . 1-11 111 



brane . tend themselves, forward, backward, and laterally, 



until they approach each other at a point over 

 the back of the foetus (Fig. 208), which is termed the "amniotic 

 umbilicus." Their opposite edges afterward actually come in con- 

 tact with each other at this point, and adhere together, so as to 

 shut in a space or cavity (Fig. 208, b) between their inner surface 

 and the body of the foetus. This space, which is filled with a clear 

 fluid, is called the amniotic cavity. At the same time, the intestinal 

 canal has begun to be formed, and the umbilical vesicle has been 

 partially separated from it, by the constriction of the abdominal 

 walls on the under surface of the body. 



There now appears a prolongation or diverticulum (Fig. 208, c) 

 growing out from the posterior portion of the intestinal canal, and 

 following the course of the amniotic fold which has preceded it ; 

 occupying, as it gradually enlarges and protrudes, the space left 



