236 TORREY 



tary cells have as true a vestigial significance as the develop- 

 ment of a tooth germ in a whale embryo." With this conclu- 

 sion, too, the facts, as we find them in TJialassema^ fall in line. 



It seems possible that not only the complete reversions to 

 radial symmetry are ancestrally reminiscent of a former condi- 

 tion, but also that the radially arranged rudimentary cells may 

 be reminiscent of the foundations of certain radial organs. 



That these cells may have once functioned as mesenchyme is 

 indicated by their behavior, by the fact that the mesoblast has 

 a radial origin in the polyclades, as has also, in a measure, the 

 ectomesoblast of certain annelids and molluscs {Crepidula, Po- 

 darke, Thalassemd), and lastly, by the fact that the cell which 

 produces all the ectomesoblast in Unio (2^2:2. i^.) is very minute in 

 AmpJiitrite and entirely rudimentary in Thalasserna. As bilat- 

 erality has been acquired, the function of producing all the ecto- 

 mesoblast has devolved on other cells. This whole process 

 is but one chapter in the history of the development of deter- 

 minate cleavage which finds its highest expression in the telo- 

 blastic development of oligochaetes and leeches. Finally, the 

 retention of these rudimentary cells in cleavage and the fact 

 that they occur in exactly the same places in several forms, in- 

 dicates that the cell itself is of greater importance in the process 

 of differentiation than some recent writers have been willing to 

 give it credit. 



Columbia University, 

 June, 1902. 



LITERATURE REFERRED TO 

 Bergh, R. S. 



'90 Neue Beitrage zur Embryologie der Anneliden. i. Zur 

 Entwicklung und Differenzirung des Keimstreifens von 

 Lumbricus 



Zeitsch. f. wiss. ZooL, Bd, L 

 Brooks, W. K. 



'80 The Development of the Oyster 



Studies Biol. Lab. J. Hopkins Univ.., Vol. I 

 Burger, 0. 



'91 Beitrage zur Entwicklungsgeschichte der Hirudineen. Zur 

 Embryologie von Nephelis 

 Zool. Jahrh., Bd. IV 



