process of gradual separation of the embryo from the yolk, 

 "as if an unseen hand outbalances the places of communication 

 among the embryo, embryonic membrane and yolk" (I 5f, p. 86 

 (47)). In this case the protrusion of the mucous and serous 

 layers takes place, and also the growth of the ends of the 

 digestive canal. 



At the same time as the described separation, the 

 external edges of the lateral plates form the fold extending 

 above the surface of the blastoderm, which spreads into the 

 peripheral part of the blastoderm at first under a blunt, then 

 under a straight, and finally under an acute angle. At the 

 cephalic and caudal ends, this fold appears earlier and 

 gradually becomes sharper. Baer considered that Wolff's 

 term "false amnion" referred especially to this ring of the 

 common fold. 



The true amnion forms, according to Baer, from the serous 

 layer (he gave it the still current name "amniotic fold"**). 

 The ring of this fold becomes narrow above the back of the 

 embryo, then by the end of the third day it closes, and the 

 embryo appears included in a sac which is the true amnion. 



At the time of the formation of the amnion, the head end 

 of the embryo bends increasingly downwards. At the same time 

 the head turns to the right, and gradually the whole embryo 

 lies on its left side. The lateral projections of the heart, 

 described previously (I 2q) , are now transformed into lateral 

 veins of the vascular area, and the common stem into which 

 they pour forms the posterior end of the heart. The heart 

 still has the form of a canal, which grows wide and bends a 

 bit to the left, then strongly to the right and down. This 

 canal continues to narrow, then moves left and upwards, where 

 it divides into four pairs of arches. Between the vascular 

 arches, the substance of the abdominal plates becomes thin, 

 and three pairs of slits appear, leading from the outside into 

 the digestive cavity or the future gullet. The crescent-shaped 

 space between the slits Baer called "the branchial arches" 



8. In fact the amniotic fold is composed from the ectoderm 

 and the parietal layer of the mesoderm. 



322 



