1696 



HUMAN ANATOMY. 



Its migration to the ventral surface is associated with the growth and changes affect- 

 ing the tract situated between the neurenteric canal and the anal anlage giving rise 

 to the tail-bud (Hertwig) from which the caudal appendage arises. In conse- 

 quence of the displacement occasioned by these changes, the anal anlage gradually 

 assumes a ventral position immediately beneath the tail. 



Coincident with this migration the primitive gut-tube becomes enlarged in the 

 vicinity of the allantois to form a common space, the cloaca, into which open the hind- 

 gut, the allantois, the Wolffian ducts, and the caudal or post-anal gut, a temporary 

 entension of the gut-tract toward the tail-bud. The ventral wall of the cloaca shutting 

 it off from the exterior is formed by a delicate partition, the anal or cloaca! mem- 

 brane (Fig. 1644), consisting of the apposed entoblast and ectoblast. A slight de- 

 pression, the primitive anal groove, indicates the position at which the membrane 

 breaks through to establish the cloacal orifice in those forms, as birds and mono- 



Optic vesicle 

 Fore-brain 



FIG. 1430. 



.Mid-brain 



.Hind-brain 



I pharyng. pouch 



_ Ventral aorta 



J--[I pharyng. pouch 



I III pharyng. pouch 



-Gut-tube 



Duct of Cuvier 



i aortic bow 



Aorta 



Neural tube 



Gut-tube, 

 lower part 



Vitelline 



duct 

 Vitelline 



artery 



Vitelli. 

 duct 



Allantoic duct 



Belly-stalk 



Reconstruction of sagittally sectioned human embryo 

 of third week, showing relations of digestive tube. X 26. 

 (After His model.) 



1 pharyng. pouch 



2 aortic bow 



II pharyng. pouch 

 3 aortic bow 



III pharyng. pouch 

 4 aortic bow 



IV pharyng. 

 pouch 



Lung-anlage 



Allantoic duct. 

 Umbilical artery_ 



Reconstruction of digestive tube of preceding em- 

 bryo ; aortic bows and trunk also shown. X 26. (After 

 His model.) 



tivmes, in which the cloaca persists. In the higher mammals the cloacal stage is 

 only temporary, the cloaca becoming subdivided into two compartments by the for- 

 mation of a septum, which grows downward to meet the cloacal membrane. The 

 anterior compartment becomes the uro-genital sinus, the posterior the rectum. 

 Later the remains of the cloacal membrane disappear, and these spaces are provided 

 with the uro-genital cleft and the definitive anus respectively. 



Differentiation <>f the simple gut-tube into distinctive segments begins with the 

 stomach, which appears as a small spindle-form enlargement at some little distance 

 below tin- primitive pharynx, the portion of the- tube between the two correspond- 

 in- to tlie early <i-soplui>us. Thr gut-tube lies close to the posterior wall of the 

 ity, and at this sta-e ( corresponding to about the fourth week in the human 

 embryo) presents live divisions, the primitive oral cavity, the primitive pharynx, 

 tlie u'sophagus, the stomach, and the intestinal tube, which latter freely communi- 

 cates with the yolk sac through the vitellinc duct. 



