A 74 MM. POLYODON. 



C. H. DANFORTH. 

 (From the Anatomical Laboratory of Washington University.) 



The developmental stages of Polyodon have long been sought 

 by biologists but, so far as published accounts show, neither the 

 fertilized eggs nor the young embryos have ever been seen. 

 For this reason it seems advisable to announce the capture of a 

 specimen smaller than any previously recorded. 



The individual in question was taken from the Mississippi 

 River near St. Louis on July 12, 1910. It was immediately 

 killed and fixed in Zenker's fluid and hardened in alcohol. In the 

 latter medium it measured 74 mm. from the tip of its snout to 

 the tip of its tail. The figures show three views of this embryo 

 after preservation. 



The features in which it seems to differ most from the adult 

 are the relatively large barbels, the rostrum, and the fins and tail. 

 The general outline of the body, too, appears rather more fusi- 

 form, but a number of measurements failed to bring out any 

 marked peculiarities in this respect. 



The barbels, represented in Figs. I and 2, are approximately 

 a millimeter long, or .013 of the total length of the fish. This is 

 relatively many times their adult size. They are, however, small 

 and apparently rudimentary even in the young, and a macro- 

 scopic examination of them revealed no new points of interest. 

 In fish of 170-175 mm. they are still relatively large as compared 

 with the adult. 



The rostrum of the embryo is somewhat unlike that of the 

 adult in outline. In the former (cf. Figs. 2 and 3) it tapers in 

 width from the nostril to the tip with only a slight constriction 

 near the middle, whereas in the adult there is a characteristic 

 and generally well marked constriction of the proximal half and 

 usually a distinct dilation of the distal half, giving the whole its 

 typical paddle-shaped appearance. In an embryo of 89 mm. 

 the rostrum is similar to that of the 74-mm. specimen, but in 



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