CHAPTER XXI. DEVELOPMENT OF THE SYSTEMS 

 OF ORGANS.* 



1. The Alimentary Canal StoniodsRum. In all Ver- 

 tebrata the beginning and end of the alimentary canal are 

 lined by epiblast instead of hypoblast, and originate in- 

 dependently of and later than the main, hypoblast-lined 

 portion, or mesenteron. The most 

 interesting structures derived from 

 the stomodseum are the pituitary and 

 the teeth. 



Of the former we need only say that it 

 arises as a mass of epiblast cells in the 

 dorsal region of the stomodseum, and at 

 once becomes connected with the floor of 

 the primitive fore-brain. As mesoblastic 

 tissues increase in amount in this neigh- 

 bourhood they cut off its connexion with 

 the stomodreum. Too little is understood 

 of the meaning of this peculiar develop- 

 ment to make it worth while to say more. 



Fig. 141. DEVELOPMENT 

 OF TEETH. 



tebrate with a constant 

 succession of teeth (e.g. 

 dogfish), three enamel- 

 organs with rudimentary 

 teeth being seen, besides 

 the functional tooth. 



The first sign of the development of 

 teeth is a series of downgrowths of 

 the epiblast into the mesoblast be- 

 neath, called enamel-organs (fig. 141). 

 Each of these becomes greatly enlarged 

 and nearly separated from the epiblast. 

 A very vascular mass of mesoblast 

 then indents it, and gives rise to the 



dentine ; the enamel being secreted from the enamel-organ. 

 In the case of those teeth of the rabbit which are repre- 



* The development of all the essential parts of the great systems 

 is so similar in frog, chick, and rabbit, that it is convenient to 

 describe it in general terms, mentioning the special types only where 

 important differences occur between them. 



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