580 TEXT-BOOK OF EMBRYOLOGY. 



formation of a complete amnion and amniotic cavity in such forms as the bat, 

 mole and man. 



The human amniotic cavity is at first small, the amnion covering only the 

 dorsum of the embryo to which it is closely applied. The dorsal surface of the 

 disk is at first concave, then flat, and later its margins curve ventrally as the flat 

 disk becomes transformed into the definite shape of the embryonic body. As 

 the margins of the disk bend ventrally they carry with them the attached amnion. 

 As the embryo becomes constricted off from the yolk sac, the amnion is attached 

 only ventrally in the region of the developing umbilical cord. With the 

 exception of this attachment the embryo thus comes to lie free, floating in 

 the amniotic fluid (Fig. 487, 6). 



The amniotic cavity, at first small, increases rapidly in size and by the third 

 month has reached the limits of the chorionic vesicle completely filling it. It 

 then attaches itself loosely to the overlying chorion thus completely obliterating 

 the extraembryonic body cavity. The amnion consists everywhere of two 

 layers, an inner ectoderm, the cells of which are at first flat, later cuboidal or 

 even columnar, and an outer layer of somatic mesoderm. At the dermal 

 navel (p. 569) the amniotic ectoderm is continuous with the surface ectoderm 

 (later epidermis) of the embryo. Some writers consider the fact that the 

 epithelial covering of the umbilical cord is stratified as indicating that it is 

 derived from embryonic ectoderm rather than from amniotic ectoderm, and 

 describe the transition between the two as taking place not at the dermal 

 umbilicus but at the attachment of the cord to the placenta. As in lower 

 forms (p. 566) the walls of the amniotic cavity contain contractile elements 

 which determine rhythmical contractions of the amnion. 



The human amniotic fluid is a thin, watery fluid of slightly alkaline reaction 

 containing about one per cent, of solids, chiefly urea, albumin and grape- 

 sugar. The origin of the fluid is not known. By some it is believed to be 

 mainly a secretion of the maternal tissues, by others as largely of fcetal origin. 

 The urea it contains is probably excreted by the fcetal kidneys. 



When the amount of amniotic fluid is excessive the condition is known as 

 hydramnios. If, as is sometimes the case, the amniotic fluid is present in very 

 small amount, adhesions may form between the amnion and the embryo. 

 These may result in malformations. With or without abnormality in the 

 amount of amniotic fluid, bands of fibrous tissue may stretch across the cavity. 

 If sufficiently strong these may produce such malformations as splitting of 

 a lip or of the nose, or the partial or complete amputation of a limb. 



In labor a portion of the amnion filled with fluid usually precedes the head 

 through the cervical canal. It is rounded or conical, and becoming distended 

 and tense with each uterine contraction or labor pain, serves as the natural 

 and most efficient dilator of the cervix. When the cervix is partially or com- 



