THE LIMBS. 39 



of the embryo, but as the embryo rapidly extends, in all directions, from the 

 margin of the orifice, the latter soon becomes relatively small. Ultimately the 

 various parts of the margin of the orifice are approximated until they fuse together, 

 closing the opening and forming a cicatrix on the ventral wall of the abdomen 

 which is known as the umbilicus or navel. 



THE EMBEYO. 



Whilst the embryonic area is being folded into the form of the embryo, the 

 neural groove on the surface of the area is being converted into the neural tube. 

 After the neural tube is completely closed and separated from the surface, during 

 the third week, the embryo is an elongated organism possessing a larger cephalic 

 end, a smaller caudal end, attached by the body stalk to the chorion (Fig. 49), a 

 continuous and unbroken dorsal surface, a ventral surface separated into cephalic 

 and caudal portions by the umbilical orifice, two lateral surfaces right and left, and 

 it contains within its interior three cavities : (1) The cavity of the neural tube, 

 which becomes the cavities of the brain and the spinal medulla (Fig. 50) ; (2) the 

 primitive alimentary canal, which is a portion of the entodermal vesicle constricted 

 off during the folding of the embryonic area (Figs. 37, 40); (3) the embryonic 

 ccelom. The coelom consists of right and left portions which communicate at 

 the margin of the umbilicus with the extra-embryonic coelom, and with each other 

 through the pericardial portion of the intra-embryonic ccelom in the ventral wall 

 of the fore-gut of the embryo (Figs. 49, 90). 



At this period the embryo is easily distinguished from the remainder of the 

 zygote, and it is so far developed that indications of its general plan of organisa- 

 tion are discernible. 



It has, as yet, no limbs, but the general contour of the head and body are 

 defined. It possesses a notochord or primitive skeletal axis, afterwards replaced 

 by the permanent vertebral column. On the dorsal aspect of the notochord lies 

 the neural tube, which is the rudiment of the future brain and the spinal medulla. 



At the sides of the neural tube and the notochord are the mesodermal somites 

 and the nerve ganglia (Figs. 40, 43). 



Ventral to the notochord is the primitive alimentary canal (Fig. 50), closed at 

 its cephalic end by the bucco-pharyngeal membrane, and at its caudal end by 

 what was originally the caudal portion of the primitive streak, but which is now 

 called the cloacal membrane because it separates the caudal end of the hind-gut, 

 which becomes the entodermal cloaca, from the amniotic cavity (Fig. 50). 



At the sides of the primitive alimentary canal are the right and left lateral 

 parts of the coelom, and between the dorsal angle of each half of the coelom and 

 the mesodermal somites of the same side lies the intermediate cell tract which is 

 the rudiment of the greater part of the genito-urinary system (Figs. 39, 40). 



Ventral to the fore-gut is the pericardial mesoderm, traversed by the pericardial 

 portion of the ccelom, which is connected dorsally, on each side, with the corre- 

 sponding lateral portions of the coelom ; and ventral to the hind-gut is the cloacal 

 membrane. Between the pericardial region at the one end and the cloacal 

 membrane at the other lies the umbilical orifice, through which the mid-gut 

 communicates with the yolk sac, the intra-embryonic part of the coelom with 

 the extra-embryonic coelom, and the allantoic diverticulum with the cloaca (Figs. 

 39, 50). 



THE LIMBS. 



When it is first defined the embryo is entirely devoid of limbs (Fig. 51). 

 During the third week a superficial ridge appears on each side, along the line 

 of the intermediate cell tract in the interior. This is the Wolffian ridge, and 

 upon it the rudiments of the fore and hind limbs, the limb buds, are formed, 

 as secondary elevations ; the fore-limb buds preceding the hind-limb buds in time 

 of appearance (Fig. 52). 



