i626 A MANUAL OF ANATOMY 



from the somatic mesoderm of the embryo, as well as from the meso- 

 derm of the allantoic or body-stalk. The embryonic bud lies- 

 originally below the amniotic cavity. As the periphery of the bud 

 curves downwards or ventralwards, it carries with it the wall of the 

 amnion where that is connected with the periphery of the bud. 

 The roof of the amniotic cavity is formed by the superficial cells of 

 the inner cell-mass, covered by that portion of the enveloping layer 

 which is known as Rauber's layer. The floor is formed by the 

 embryonic bud or disc. The cells on the amniotic surface of this 

 bud spread out over the wall of the cavity and become continuous 

 with the roof-cells derived from the inner cell-mass, forming the 

 embryonic ectoderm, as distinguished from the extra-embryonia 

 ectoderm, which is represented by the enveloping layer. The 

 embryonic ectoderm thus lines the amniotic cavity, and it becomes 

 continuous with the extra-embryonic ectoderm or enveloping layer 

 at the periphery of the roof of the amniotic cavity, that part of the 

 enveloping layer which forms Rauber's layer having disappeared. 

 External to the lining layer of embryonic ectoderm there is a layer 

 of somatic mesoderm, which is derived from the somatic mesoderm 

 of the embryo, as well as from the mesoderm of the allantoic or 

 belly-stalk. The wall of the amniotic cavity therefore consists of 

 two layers — an inner, formed by embryonic ectoderm ; and an outer, 

 formed by somatic mesoderm — both connected together. 



The embryonic bud or disc, as stated, lies originally in the floor of 

 the amniotic cavity. As the periphery of the bud curves ventral- 

 wards, it carries with it the wall of the amnion, where that is con- 

 nected with the periphery of the bud. 



At the caudal or posterior extremity of the embryo the inner and 

 outer layers of the amnion still remain connected. In. this region, 

 the outer layer of the amnion, formed by somatic mesoderm, is 

 continuous with the inner layer of the chorion, which also consists 

 of somatic mesoderm. This connection gives rise to the allantoic 

 or belly-stalk, and, as the amnion is continuous with the body-wall 

 of the embryo, the allantoic stalk connects the embryo with that 

 part of the chorion which subsequently forms, along with the meso- 

 derm of the allantoic stalk, the foetal part of the placenta. 



The amnion is peculiar to Mammals, Birds, and Reptiles, these three classes 

 of Vertebrata forming the group of Amniota. In Amphibia and Fishes the 

 amnion is absent, and these two classes are consequently grouped as 

 Anamnia. 



Pro-Amnion. — In front of the embryonic area of the blastodermic vesicle 

 is the pro-amniotic area, the blastoderm, of which is bilaminar, the two layers 

 consisting of ectoderm and entoderm. As the head of the embryo grows 

 forwards, it comes to lie over the pro-amniotic region, and the embryo is 

 meanwhile sinking slightly. The bilaminar blastoderm of the pro-amniotic 

 region thus forms a hood for the head of the embryo, this hood being known 

 as the pro-amnlon. At a later period the pro-amnion is invaded by somatic 

 and splanchnic lamellae of mesoderm, and then it consists of somatopleure 

 and splanchnopleure. Its somatopleuric constituent fuses with that division 

 of the amniotic folds which is called the head-fold, and therefore forms part 

 of the amnion. The pro-amnion is well marked in the chick, but in the 

 human embryo, if present at all, it is only for a very brief period. 



