1624 A MANUAL OF ANATOMY 



embryo become bent ventralwards, and these bent ends form the 

 cephalic or head-fold and the caudal or tail-fold. Posteriorly the 

 hind-gut is, at this stage, closed by the cloacal membrane, which, 

 like the bucco-pharyngeal membrane at the cephalic end of the ali- 

 mentary tract, consists of two layers — an outer or ectodermic, and 

 an inner or entodermic. The entire primitive intestinal canal is 

 therefore closed at either end, cephalic and caudal, by bilaminar 

 membranes — ^bucco-pharjmgeal and cloacal — the internal lamina of 

 each of which consists of entoderm. 



The wall of the primitive intestinal canal is formed by splanchno- 

 pleure — that is to say, entoderm and the splanchnic lamella of the 

 lateral plate of mesoderm — ^the tube being lined with entodermic 

 cells. 



The unfolding processes, just described, accomplish the separation 

 of the embryonic area on the dorsal aspect of the blastodermic 

 vesicle, and that area now represents the primitive embryo. At this 

 stage the embryo consists of three tubes and a solid cylinder of cells. 

 The tubes are as follows: (i) The neural tube; (2) the primitive 

 intestinal canal; and (3) the coelom, body-cavity, or pleuro-peri- 

 toneal cavity. The solid cylinder of cells is the notochord. The 

 order of these component parts dorso -ventralwards is as follows: 



Neural tube or canal. 



Notochord. 



Primitive intestinal canal, enclosed by splanchnopleure. 



Coelom or pleuro-peritoneal cavity, enclosed by somatopleure. 



The Appendages of the Embryo. 



The appendages or supplementary organs of the embryo are, for 

 the most part, developed from the extra-embryonic area of the 

 blastodermic vesicle, and they are as follows : 



1. The amnion. 



2. The yolk-sac and vitelline duct. 



3. The chorion (representing a large part of the placenta). 



4. The allantois (diverticulum and stalk). 



5. The umbilical cord. 



Functionally they are (i) protective, (2) nutritive, (3) respiratory, 

 and (4) excretory. All these appendages are dispensed with at 

 birth, the umbilical cord being then divided. 



The Amnion. 



The amnion is one of the two fceial deciduous membranes, the other 

 being the chorion. It is a closed membranous sac, the cavity' of 

 which, called the amniotic cavity, contains a clear albuminous fluid, 

 known as the liquor amnii. The amnion surrounds the embryo, 

 which floats in the liquor amnii. When this fluid is present in ex- 

 cessive quantity, the condition is known as hydramnios, or dropsy 

 of the amnion. Rupture of the amnion and escape of the liquor 

 amnii usually precede the birth of the child. 



