FORMATION OF THE MEMBRANES. 



363 



The two amniotic layers are formed in the way that we 

 have just described, and a complete separation finally takes 

 place, by a disappearance of the septum 

 formed by the meeting of the folds over 

 the back of the embryon. According to 

 Longet, this process occupies four or five 

 days in the human ovum ; * though the 

 observation by Allen Thomson shows 

 that the complete separation of the two 

 layers occurs much later. The point 

 where the folds meet is called the am- 

 niotic umbilicus. When the amnion is 

 thus completely formed, the vitelline 

 membrane has been encroached upon by 

 the external amniotic layer and disappears, leaving this layer 

 of the amnion as the external covering of the ovum. At 

 this time, there is a grow r th of villosities 

 upon the surface of the external amni- 

 otic layer, which, like the villosities of 

 the vitelline membrane, are not vascular. 



Soon after the development of the 

 amnion, the allantois is formed. This 

 membrane is vascular, encroaches upon 

 and takes the place of the external am- 

 niotic membrane, becomes villous, and 

 its villosities take the place of those of 

 the amnion. Over a certain portion of 

 the membrane, the villosities are per- 

 manent. The mode of development of 

 the amnion, as we have described it, was 

 discovered by Yon Baer, a and is illus- 

 trated by the diagrammatic Figs. 34 and 

 35. These figures show the formation of the amnion, the 



Fecundated egg. 'a, umbili- 

 cal vesicle: &, amniotic 

 cavity : c, allantois. (DAL- 

 TOK, Human Physiology, 

 Philadelphia, 1871, p. 621.) 



FIG. 85. 



Fecundated egg, with allan- 

 tois nearly complete. a, 

 inner lamina of amniotic 

 fold ; &, outer lamina of 

 ditto ; c, point where the 

 amniotic folds come in 

 contact. The allnntois is 

 seen penetrating between 

 the inner and outer lami- 

 na? of the amniotic folds. 

 (DALTON, Human Physi- 

 olo(jj/, Philadelphia. Ib71, 

 p. 621.) 



1 LONGET, Traite de physiologic, Paris, 1869, tome in., p. 844. 



2 VON BAER, Ueber Entwicklungsgeschicte der Thiere, Konigsberg, 1837, 

 Zweite Theil, S. 192. 



