THE EVOLUTION OF MAN. 



shaped organ, in which the food is especially dissolved and 

 digested, is not so complex in structure in the lower Verte- 

 brates as in the higher. Thus, for instance, in many Fishes, 

 it appears as a very simple spindle-shaped expansion at the 



FIG. 286. Longitudinal section through an embryonic Chick on the 

 fifth day of incubation : d, intestine ; o, month ; a, anus ; I, lungs ; h, livei ; 

 q, mesentery ; v, auricle of heart ; k, ventricle of heart ; b, arterial arches j 

 I, aorta; c, yelk-sac; m, yelk-duct; u, allantois; r, stalk of allantois; n, 

 amnion ; w, amnion-cavity ; s, serous membrane. (After Baer.) 



beginning of the digestive section of the intestine, which 

 latter passes from front to rear in a straight line under the 

 spinal column in the central plane of the body. In Mam- 

 mals the rudiment of this organ is as simple as it thus 

 is permanently in Fishes . but at a very early period the 

 various parts of the stomach-sac begin to develop unequally. 

 As the left side of the spindle-shaped pouch grows much 

 more vigorously than the right, and as, at the same time, 



