170 



drücklich bemerken, daß die beiden Pharyngealbildungen stets von 

 vornherein auf die Ventralseite zu liegen kommen.« 



Save for the minor point that the second pharj'nx appears just be- 

 hind instead of actually within the area occupied by the embryonic struc- 

 ture, Mattiesens observations on this point are a confirmation of what 

 Ijima^ thought was the relation in Dendrocoelum lacteum^ though 

 Matties en has rendered his description quite conclusive by the care 

 with which he has studied and figured the histological details at each 

 stage of the development. The determination of the exact position of 

 these two organs in P. torva was made easy by the fact that the degene- 

 rating embryo-pharynx still persists at a stage when the anläge of the 

 adult pharynx is clearly recognizable, whereas I j ima found that in D. lac- 

 teum the embryo-pharynx entirely disaj^peared before the first sign of the 

 adult organ could be detected. 



At the close of his description, Mattiesen notes that I have de- 

 scribed and figured 3 (see fig. 1 of this paper) the embryonic pharynx of the 

 American Planaria maculata as located on the future dorsal surface 

 of the body and immediately above the point at which the adult pharynx 

 takes its origin, but he dismisses this as hardly Avorthy of serious consi- 

 deration and doubtless to be explained on the basis of poor preservation 

 or of the observation being made upon a single abnormal individual^. 



In replying to this intimation that my record of the facts in P. ma- 

 culata can hardly be taken seriously, I shall present a brief resumé of 

 the observations to date upon this point in the embryology of P. macu- 

 lata and the other planarians in which it has been definitely established. 

 I will then state what in my opinion is the view that should now be taken 

 of this particular feature in planarian embryology. 



Ijima^ who first gave an approximately correct account of the 

 origin of the permanent phar3nix studied the embryo of Dendrocoelum 

 lacteum. After describing the formation of the spherical embryo and 

 its distention by the yolk cells which are taken through the pharynx of 



2 J. Ijima, Untersuchungen über den Bau und die Entwicklungsgeschichte der 

 Süßwasserdendrocölen. Zeitschr. f. wiss. ZooL XL. Bd. 1884. 



3 Curtis, The LifeHistorj^ etc. oî Planaria maculata. Proc. Boston Soc. Nat. 

 Hist. Vol 30, No. 7, p. 515—559, PI. 9—19. (See fig. 51 of plate 17.) 



4 »Es wundert mich daher sehr, daß die von Curtis beschriebene P. maculata 

 hiervon eine Ausnahme machen soll. Dieser amerikanische Autor glaubt nämlich 

 gefunden zu haben , daß der sich bildende definitive Pharynx tatsächlich auf die 

 Ventralseite des Embryo zu liegen kommt, während der degenerierende Embryonal- 

 pharynx, der in diesem Falle sogar hinter dem definitiven liegen soll, sich dorsal 

 am äußersten Hinterende befindet. Curtis bildet auch einen derartigen Embryo im 

 Längsschnitt auf seiner Fig. 51 ab. Ich halte es jedoch für sehr möglich, daß dieser 

 entweder durch die Konservierung stark deformiert oder vielleicht einfach mißge- 

 bildet ist, was eine richtige Orientierung ausschließt.« 



5 op. cit. 



