210 EMBRYOLOGY. 



of the fold is carried farther and farther from the s^tlanchnopleure 

 (sp), which remains spread out flat over the yolk. In this way the 

 extra-embryonic part of the body-cavity, or the cavity of the blasto- 

 derm (KOLLIKEB), increases in extent in the vicinity of the embryo. 

 'When the lateral folds of the amnion have grown up to the dorsal 

 surface of the embryo (Plate I., fig. 9 saf), they begin, by the bending 

 over of their edges rnedianwards, to form the so-called lateral 

 sheaths. 



Inasmuch as the folds of the amnion, which are called by special 

 names, become, when they are in full development, continuous, and are 

 only parts of a single ring like fold, the embryo eventually becomes 

 surrounded on all sides as though by a high wall. With further 

 enlargement, the amniotic sheaths then bend together over the back 

 of the embryo from in front and behind, and from the right and 

 the left (Plate I., figs. 2, 3, and 10, af, vaf, haf), come together 

 with their edges in the median plane, and then fuse with each other 

 along a line, the amniotic suture, which closes from in front back- 

 wards (Plate I., fig. 10), except that at one very small place near 

 the tail-end the closing is interrupted for a considerable time, and 

 a small opening is preserved. 



The fusion of the amniotic folds takes place in the same manner 

 as the fusion of the medullary folds described on page 79. Each 

 fold (Plate I., figs. 3 and 10) consists of two layers, an inner and an 

 outer one, which are continuous at the margins of the folds, and are 

 separated by a fissure, which is a portion of the extra-embryonic 

 body-cavity. At the amniotic suture the corresponding layers of 

 the folds of both sides fuse, and hand in hand with this a separa- 

 tion of the inner from the outer layers takes place (Plate I., fig. 4). 

 As a result of this there have now arisen two envelopes over the 

 back of the embryo, an inner and an outer one, the amnion (A) and 

 the serosa (S). 



The amnion is the product of the inner layer of the folds (Plate I., 

 fig. 10 ifb). It forms a sac which immediately after its origin is 

 closely applied about the embryo, and which encloses a very small 

 amniotic cavity filled with fluid. 



The serous membrane (serosa), which is derived from the outer 

 layer of the folds (a/6, Plate I., fig. 10), lies as a very delicate trans- 

 parent membrane closely applied to the amnion, and thus encloses 

 the embryo in still another envelope. 



If we now glance back at the conditions described in the previous 

 chapter, and compare the development of Fishes with that of Reptiles 



