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-biid ( Hertwig j from which the caudal appendage arises. In conse- 

 quence of the displacement occasioned by these changes, the anal anlage graduall) 

 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 cloacal mem- 

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

 pression, t\\e: primitive anal groove, indicates the position at which the membrane 

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



FiG. 1430. 



Optic vesicle i— 



Fnrp-hrain 1 \ 

 Oro-pharynx 



Truncus 

 arteriosus 



Priniitive_|. 

 ventricle 



Sinus 

 reuniens 



Liver 

 Liver- 

 diverticulum 



VitellineJU 



duct \ 





Allantoic duct- 



Belly-stalk. 



Mid-brain 



Hind-brain 



I pharyng. pouch 



Ventral aorta 



4~II pharyng. pouch 



III pharyng. pouch 

 'i — Gut-tube 



Duct of Cuvier 



Aorta 



Liver 



rrii iJ UMuJ 



V 



Neural tube 



Gut-tube, 

 lower part 



Vitelline 



duct 

 Vitelline 



artery 



\. 



Reconstruction of sagittally sectioned hutnan embryo 

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

 (After His mode I.) 



1 pharyng. poucii • 



2 aortic bow 



II pharyng. pouch 

 3 aortic bow 



III pharyng. pouch 

 4. aortic bow 

 I\' pharyng. 

 pouch 



Lung-anlage 



Allantoic ducc. 

 Umbilical artery- 



Reconstruction of digestive tube of preceding em- 

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

 His model. ) 



tremes, 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 cloac;3,l 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 of the simple gut-tube into distinctive segments begins with the 

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

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

 ing to the early oesophagus. The gut-tube lies close to the posterior wall of the 

 body-cavity, and at this stage (corresponding to about the fourth week in the human 

 embryo) presents five divisions, — the primitive oral cavity, the primitive pharynx, 

 the oesophagus, the stomach, and the intestinal tube, which latter freely communi- 

 cates with the yolk-sac through the vitelline duct. 



