42 DEVELOPMENT OF THE F03TAL MEMBRANES. 



DEVELOPMENT OF THE FCETAL MEMBRANES; ATTACHMENT 

 OF OVUM TO UTERUS. 



Having thus sketched out the manner in which the principal organs of the 

 body first make their appearance, we may briefly consider the formation of certain 

 structures which have a purely embryonic existence, and are concerned either with 

 the nutrition of the foetus and its attachment to the mucous membrane of the 

 uterus (chorion, allantois, placenta), or serve the purpose of protecting the embryo 

 against mechanical injuries by suspension in a bag of fluid (amnion). 



Formation of the amnion and chorion. The amnion, which is only found 

 in reptiles, birds, and mammals (amniota), is a membranous bag occupied by a clear 

 albuminous fluid, and covers the whole of the embryo. It is developed from folds of 



-false ccMntions 



-uittb of 



Fig. 44. DIAGRAM OP A TRANSVERSE SECTION OP A MAMMALIAN EMBRYO SHOWING THE MODE OP 



FORMATION OP THE AMNION. THE AMNIOTIO POLDS HAVE NEARLY UNITED IN THE MIDDLE LINE. 



Epiblast, blue ; mesoblast, red ; hypoblast and notochord, black. 



somatopleure, which are reflected from the head and tail ends and lateral boundaries 

 of the embryo at an early stage. 1 With the sinking of the embryo into the blasto- 

 dermic vesicle or yolk, these folds grow up over the back (fig. 44) until they meet and 

 coalesce with one another along the middle line, in such a manner as to form two 

 distinct membranes, one of which, the inner, is the amnion (true amnion), while the 

 outer membrane (termed the false amnion) becomes applied to the greatly thinned 

 remnant of the zona pellucida, and eventually forms a complete external covering 

 to the ovum and its contents. This external covering of the ovum has been long 

 known as the chorion a name which has, however, been applied by some embry- 

 ologists to other structures. 3 It is fixed to the uterus by villi, which are 



1 The head fold is preceded at a yet earlier stage by the bilaminar pro-amnion, the formation of 

 which, has been already alluded to (p. 35). 



2 The term " chorion" has been applied to various structures by embryologists. Originally used to 

 denote the external covering of the developing ovum, it was employed successively for the zona pellucida 

 (" primitive chorion "), the epithelial enclosing membrane of the blastodermic vesicle, and finally for 

 the external amniotic fold or false amnion, when this becomes formed. Lately it has been used to 

 express the external albuminous envelope of the undeveloped ovum, so that it is probable that 

 much confusion may arise unless the meaning of the term be in each case clearly denned. It 



