THE NOTOCHORD AND ALIMENTARY CANAL 445 



the formation of the vertebrate alimentary canal was a repetition of 

 the same process which had already led to the formation of the 

 notochordal tube. The formation of the anterior part of the ali- 

 mentary canal in Ammoccetes at the time of transformation strongly 

 suggests the marked similarity of the two processes. 



Of all the startling surprises which occur at transformation, this 

 formation of a new anterior gut is the most startling. From the 

 oral chamber of Petromyzon two tubes start : the one leads into the 

 gill-chambers, is known as the bronchus, and is entirely concerned 

 with respiration ; the other leads without a break from the mouth 

 to the anus, has no connection with respiration, and is the alimentary 

 canal of the animal. Any one looking at Petromyzon would say 

 that its alimentary canal was absolutely non-respiratory in character. 

 Before transformation, this kind of alimentary canal commences at 

 the end of the respiratory chamber ; from here to the anus it is of 

 the same character as in Petromyzon, but in Ammoccetes the non- 

 respiratory anterior part simply does not exist : the whole anterior 

 chamber is both respiratory and affords passage to food. This part 

 of the alimentary canal of the adult is formed anew. We see, then, 

 here the formation of a part of the alimentary canal taking place, not 

 in an embryo full of yolk, but in a free-living, independent, grown-up 

 larval form in which all yolk has long since disappeared : a condition 

 absolutely unique in the vertebrate kingdom, but one which more 

 than any other may be expected to give a clue to the method of 

 formation of a vertebrate gut. 



The formation of this new gut can be easily followed at trans- 

 formation, and was originally described by Schneider. His statement 

 has been confirmed by Nestler, and its absolute truth has been 

 demonstrated to me again and again by Miss Alcock, in her specimens 

 illustrative of the transformation process. First, in the mid-dorsal 

 line of the respiratory chamber a distinct groove is formed, the 

 edges of which come together and form a solid rod. This solid rod 

 blocks the opening of the respiratory chamber into the mid-gut, so 

 that during this period of the transformation no food can pass out of 

 the pharyngeal chamber. A lumen then begins to appear in this 

 solid rod at the posterior end, which steadily advances mouthwards 

 until it opens into the oral chamber and thus forms an open tube 

 connecting the mouth with the gut. 



Here, then, is the foundation of a new gut on very similar lines 



