DIGESTIVE SYSTEM 



723 



that mouth and anus are composed of ectoderm and not endoderm as is 

 the central digestive tube. After the indentation has gone far enough, 

 the thin plate of cells separating the central digestive tube from the 

 mouth and anus breaks through, so that a continuous opening is formed, 

 from the mouth through the digestive canal to the anal opening. 



All the additional structures that go to make up the digestive system 

 as well as the respiratory system are formed by inpushings or outpush- 



iii. 



Fig. 426. 



I. Diagrams to show method of outpushings in digestive tract. A, 6mm. pig 

 embryo ; B, same at 8 mm. ; C, same at 10 mm. t, trachea ; e, oesophagus ; s, 

 stomach ; I, liver ; d.p., dorsal pancreas ; v.p., ventral pancreas ; s.i., small intestine ; 

 c, caecum; v.d., vitelline duct. (From Carey, Journal of General Physiology. Vol. 

 III. No. 1). 



II. Three schematic views of variations in the ducts leading from the gall-blad- 

 der, c and s, cystic duct ; ch, ductus choledochus ; h, hepatic duct ; he, hepato- 

 cystic duct; he, hepato-enteric duct; vf, gall-bladder. (From Schimkewitsch after 

 Wiedersheim ) . 



III. A diagrammatic section of the cloaca of a male bird. (After Gadow.) 

 cd., Upper region of cloaca into which rectum opens ; ud., median region into 

 which ureter (u.) and vas deferens (vd.) open from each side; pd., posterior 

 region into which the bursa Fabricii (B.F.) opens. 



ings (Fig. 426) of this elementary digestive tract and it will be necessary 

 to remember in one's study of all the higher forms, that no matter how 

 many of these inpushings or outpushings there may develop, and no 

 matter how lengthy the digestive tube may grow T and coil, if it be 

 straightened out it will to all intents and purposes be a continuous hol- 

 low tube whose interior is really outside the body in so far as it is sub- 

 ject to all the external conditions to which the body itself is subject. 

 In other words, one may the better understand this if a hollow gas pipe, 

 open at both ends, is thought of. The hollow, straight opening, through 

 which the eye can see, represents the digestive canal. The metal of 

 which the pipe is composed represents the walls of the digestive tube. 



