88 BULLETIN OF THE 



II. Relation of the Observations on Budding in Bryozoa 

 to the Germ Layer Theory. 



No question in Bryozoan morphology has been more thoroughly dis- 

 cussed than that of the part played by the germ layers in the production 

 of the polypide, and upon none has there been less agreement. Nitsche 

 first boldly opened the question, and concluded that we have in this pro- 

 cess a fatal objection to the idea of the homology of the germ layers, in 

 so far as their homology depends upon a similarity of fate throughout the 

 Metazoa. A single layer, the invaginated ectoderm, gives rise to the outer 

 covering of the tentacles, to the pharynx, and to the brain, — structures 

 elsewhere considered as ectodermal, — and also to the lining of the ali- 

 mentary tract, elsewhere universally accounted entodermal. In view of 

 these facts, "sind die Keimblatter," concludes Nitsche ('75, p. 398), " kei- 

 neswegs mit einer besonderen histologischen Pradisposition ausgestattete 

 Zellschichten, sondern lediglich die flachenhaft ausgebreiteten Elemente, 

 aus denen die den Metazoenkorper zusammensetzenden, ineinander 

 gescliachtelten Eohren sich bilden." Prouho, although recognizing the 

 facts to be as stated by Nitsche, has not discussed the theoretical bearing 

 of the question. Seeliger ('89% p. 204) finds in the budding process of 

 Endoprocta a shortening and confusion of the embryonic process. " Wie 

 die gesammte Knospenentwicklung verkiirzt ist, erscheinen auch die 

 beiden Processe der Einstiilpung durch welche im Embryo zuerst Ento- 

 dermkanal, dann Atrium sich bilden, in einen zusammengezogen." In 

 another place (Seeliger, '90, p. 595) the budding process is considered as 

 an " immer sich erneuerende Gastrulationsvorgang." Braem ('90, p. 116) 

 regards the inner layer of the bud as entoderm, and the process of its 

 formation as one of gastrulation. In a preliminary notice published last 

 February (Davenport, '91, p. 279) I suggested that the embryonic tissue 

 from which the inner layer of the polypide arises is to be regarded as 

 " neither ectoderm nor entoderm, but as still indifferent, and capable of 

 giving rise to either." A few weeks ago I saw for the first time the paper 

 of Oka ('90), in which he offers (p. 145) a priori a similar suggestion 

 concerning the significance of the embryonic tissue from which the inner 

 layer of the polypides arises. I am pleased to find that oiu' ideas, thus 

 independently arrived at, are so fully in agreement. My idea of the re- 

 lation of the germ layers to the layers of the polypide bud chiefly grew 

 out of my studies on the embryology of Phylactolsemata as described in 

 earlier pages. 



As there are two layers to the bud, the question of the part taken hj 



