SURVEY OF HUMAN INTESTINAL SYSTEM. 



347 



IV. Fourth Period : hitestine of Skull-less Animals {AcrantCb) 

 (Fig. 282; Plate XL Fig. J 5). 

 The gill-streaks appear between the gill-openings of the res]-iratory 

 intestine ; a liver blind-sac grows from the stomach-pouch of the digestive 

 intestine ; as in the Amphioxus. 



V. Fifth Period: Intestine of Cydostonm (Plate XL Fig. IG). 

 The thyroid gland develops from the ciliated groove on the base of 

 the gills (liypobianchial groove). A compact liver-gland develops from 

 the liver blind-sac. 



VI. Sixth Period : Intestine of Primitive Fishes (p. 114). 

 Cartilaginous gill-arclies appear between the gill-openings. The fore- 

 most of these form the lip-cartilages and the jaw-skek ton (tipper and lower 

 jaw). The swimming-bladder grows from the pharynx. The veutral-salivarj 

 gland appears near tlie liver, as in Selachii. 



VII. Seventh Period : Intestine of Dipneustn (p. 118). 



The swimming-l)ladder modifies into the lungs. The mouth-cavity 

 becomes connected with the nose-cavity. The urinary bladder grows (rum 

 the last section of the intestine, as in Lepidosiren. 



VIII. Eighth Period : Intestine of Amphibia (p. 126). 



The gill-openings close. The gills are lost. The larynx originates from 

 the upper end of the trachea. 



IX. Ninth Pe^'iod : Inte^^tine of Monotremes (p. 14o). 

 The primitive mouth and nasal cavity is separated by the horizontal 

 palate-roof into the lower mouth-cavity (food passage) and the upper nose- 

 cavity (air passage); as in all Amnion Animals. 



X. Tenth Period : Intestine of Mcrsvpia'^s (p. 149). 



The existing cloaca is separated by a partition wall into an anterior 

 nrinary-sexual aperture and a posterior anal aperture. 



XL Eleventh Period: Intpstine of Catarhi'ne Ap''S (p. ITH). 

 All parts of the intestine, and especially the teeth-ap[>aiat'is, acquire the 

 characteristic development common to Man and Catarhine Apea. 



