258 ZOOLOGY. 



more and more complete body-wall. And they show the 

 development of the amnion and of the allantois, features 

 quite unknown in our two previous types, and which we 

 must now describe. 



10. The Amnion. The first sign of "the tucking-in 

 around the embryo appears towards the end of the first 

 day as a curved wrinkle in the blastoderm in front of the 

 embryo : this is called the head-fold (fig. 127). Parallel to 

 it, farther forward, is another wrinkle, the beginning of the 

 amniotic fold. Later on, wrinkles similar to the head-fold 

 appear behind the embryo (tail-fold) and on either side of it 

 (lateral folds), and each of these has a parallel amniotic fold 

 beyond it. These wrinkles all extend and eventually unite, 

 so that the embryo becomes surrounded by a pair of elliptical 

 wrinkles. The inner of these may be compared to a moat, 

 as it is essentially a depression of the blastoderm, the outer 

 (amniotic fold) to a rampart, as it is an elevation. But 

 whereas the depression concerns both body- wall (i.e. epiblast 

 plus somatic mesoblast) and mesenteron (i.e. hypoblast plus 

 splanchnic mesoblast), the elevation consists of the former 

 only. (See fig. 134, and, for a more exact representation of 

 the same stage, fig. 137). As the moat is dug deeper, it 

 also begins to curve inwards so as to undermine the embryo, 

 and separate it from the yolk-sac. As the rampart grows 

 in height it curves inwards also, so as to form a dome 

 covering over the back of the embryo. 



The amniotic fold is from the beginning hollow, con- 

 taining a space which is continuous with the general coalomic 

 space produced by the splitting of the mesoblast. Hence 

 the dome which it begins to form over the back of the 

 embryo is a double dome. The inner of the two layers, in 

 which the epiblast is turned towards the embryo and the 

 mesoblast towards the exterior, is called the true amnion ; 

 the outer layer, in which this arrangement is reversed, is 

 called the false amnion. When the dome is finally com- 

 pleted the true amnion becomes a covering for the embryo, 

 enclosing an amniotic cavity (lined entirely by epiblast) 

 which contains a secreted fluid and acts as a sort of water- 

 cushion to protect the embryo from shocks. The false 



