EVOLUTION AND DISEASE. 



and undergone gradual 

 transformation into a spinal 

 cord. These facts were 

 based on the embryological 

 history of the parts and 

 from a study of the malfor- 

 mation exhibited, not rarely, 

 by this central canal and 

 the permanent alimentary 

 canal. 



The mode of development 

 of the parts may be briefly 

 described. The alimentary 

 canal and spinal cord of all 

 vertebrates arise on a com- 

 mon plan. At a certain 

 stage it consists of a U- 

 shaped tube, as in fig. 24, 

 each limb ends in a cul-de- 

 sac y and the ventral limb is 

 connected by a hollow duct 

 with the yolk sac. 



By a series of secondary 

 changes the anterior end of 

 the ventral limb of this tube 

 is made to communicate 

 with the exterior by way of 

 the mouth and pharynx, and 



FIG. 25. A diagram representing the parts in the adult derived from the U- 

 shaped tube of the embryo. S, stomodoeum ; I, infundibulum ; Pt, pituitary 

 body; T, thyroid; L, lung; M, vitello-intestinal duct ; A, allantois ; C, 

 coccygeal body ; G, post-anal gut ; N, neurenteric passage. 



