174 



tero-posterior axis. This change in form is accomplished by a more rapid 

 cell-multiphcation in the ventral than in the dorsal body wall. The most 

 rapid cell-multiplication takes place on the postero-ventral wall just below 

 the embryo-jjharynx. It is from this mass of cells that the permanent 

 pharynx is developed." 



In another North American form, Planaria simplicissima Stevens s, 

 makes the following statement (p. 216) of the relation between these or- 

 gans. ,,The embryonic pharynx disappears completely before the adult 

 pharynx begins to form, but its relation to that structure appears to be 

 the same as in P. maculata as recently ^lescribed by Curtis. In fig. 45 

 the ventral side of the embryonic layer is easily distinguishable from the 

 dorsal side by its greater thickness, and the degenerating pharynx [p ') 

 is on the dorsal side as in Curtis's fig. 51, pi. 17". Below I have repro- 

 duced Stevens fig. 45 as figure 2 E of this paper. 



In view of these confirmations by other workers I think it un- 

 necessary to offer any further details of my own work to show that it 

 was neither hasty nor careless nor published without the examination 

 of more than one specimen. Assuming the account of the matter in 

 P. torva is correctly given by Matties en and that this indicates the 

 approximate correctness of the similar interpretation which Ijima put 

 upon the facts as he observed them im I), lacteum^ I think we may now 

 regard it as well established that the relative position of these 

 structures is not identical in all planarians, a possibility which 

 does not seem to haveoccured toMattiesen. Certainly there is a marked 

 difference in the cases above cited (P. maculata and P. simplicissima 

 as contrasted with P. torva and D. lacteum). Since such a difference is 

 known to exist in the same genus, is it not likely that a careful exami- 

 nation of this stage in a number of planarians would reveal a quite in- 

 constant relation between these organs, if indeed forms were not found 

 in which there was wide variability Avithin the limits of a single species. 

 Be this as it may, I think I have here presented data enough to sustain 

 my point that the adult jaharynx of P. maculata does not have the point 

 of origin with reference to the embryonic structure Avhich Mattiesen 

 has shown obtains in P. torva. 



While I was of the opinion in my previous paper ^ that Ijima had 

 been in error and was impressed with the idea that ,,it would be rather 

 strange to have such an entirely different relation in forms so nearly alike 

 in all their general structure as the planarians", now that this differ- 

 ence has been shown to exist beyond all question, I think it is perhaps 



i^ N. M. Stevens, On the Germ Cells and Embryology of Planaria simplissi- 

 Proc. Acad. Nat. Sciences, of Philadelphia. February 1901. 

 9 op. cit. p. 556. 



