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The problem I have to discuss is, granted that the present Verte- 

 brate mouth is a new^) structure, what traces, if any, are to be 

 found of the old mouth? 



It is conceivable, and I strongly emphazise the point, that the 

 old mouth might have disappeared, even from the development, with- 

 out leaving a trace behind. We seem to be gradually getting out of 

 the idea that ontogeny is even a fair repetition, much less a perfect 

 one, of Phylogeny, for absolutely rudimentary organs, (organs per- 

 forming no function at all) are only retained as larval or embryonic 

 organs, as the basis or "Ä.nlage" of other organs, or finally, because 

 they are inseparably connected with the development of other organs. 

 Of the latter a fair case, it seems to me, is to be seen in the rudi- 

 ment of the parietal eye in the higher Vertebrates. This absolutely 

 functionless organ , functionless except in a few fishes and reptiles, 

 possibly only reappears in the development because it is intimately 

 connected in some way or other with the paired eyes. 



A still better example is, I think, to be met with in the rudi- 

 ments of the gill sense organs and ganglia described by Prof. Froriep 

 in Mammalia. 



I shall soon give a fuller account of these in connection with 

 other work. There can be little doubt that they exist as rudi- 

 ments in all animals above fishes and amphibia, and I find them in 

 Lizards, Crocodiles and Birds. Their recurrence has its explanation in 

 that they probably form the "Anlage" for certain portions of the 

 cranial ganglia. 



It was DoHRN who first; hinted in his work on "Der Ursprung 

 der Wirbeltiere", published in 1875, that the hypophysis C(jrebri re- 

 presented the last remains of the old mouth, and that it must have 

 opened on the dorsal surface, after passing between the crura cerebri. 



This idea he soon gave up, and indeed in the work above 

 mentioned, he inclined to the view that the opening lay somewhere in 

 the region of the medulla oblongata. Since then he has relinquished, 

 for the time, the search for the old mouth, and has advised others 

 to do the same. His first hypothesis has been advocated as some- 

 thing new by Prof. Owen, and more recently by Mr. J. T. Cunningham. 

 Both of these writers hold very slightly dilferent views from those 

 originally suggested by Dohrn. Something may be here said of 

 Cunningham's hypothesis, because some of the statements I am about 



1) It is rather paradoxical to speak of a thing as new, which has 

 existed in its present form for untold millions of years. 



