ON BUDDING IN POLYZOA. 537 



passes into the ectocyst of the adult internally. Owing to the 

 rapid growth there is a large central cavity, the walls are un- 

 doubtedly lined with the syncytium and with a portion of the 

 original central blastema, while from the apex depends, icicle - 

 like, the remainder of the blastema. This latter is to form the 

 alimentary tract, its peritonenal lining and some of the muscles, 

 the rest are formed as processes from the body- wall, i.e. 

 from its inner layer. The tentacle-sheath is formed by an 

 invagination of the tissue at the apex of the bud, so that the 

 syncytium forms its inner lining; that is really the future outer 

 layer of the tentacular sheath. '' With regard to this homo- 

 geneous outer layer of the Hypophorella bud, in which one 

 would like to see the homologue of a cell-layer, I must remark 

 that I have never seen it continued into the first rudiment of 

 the gut " (p. 109). " The endoderm appears as a separate 

 development of the tissue of the indifferent body-wall at the 

 spot distinguished by the above-mentioned invagination." The 

 tentacle disc (Tentakelscheibe) or incipient lophophore is 

 formed from the endoderm. There is very early a cavity in 

 the formative material of the alimentary tract ; this is the 

 stomach. The tentacles grow out from the edge of the tentacle 

 disc, at first only eight. The remaining two or three appear 

 a little later. [It is usually stated that the permanent number 

 of tentacles arise from the first, but in several forms, e.g. Fl. 

 papyracea, Diachoris magellanica, &c., I have observed 

 that four lateral ones usually appear first, the more central 

 being the larger, but I have not yet satisfied myself as to the 

 exact rhythm ; ' lateral ' has, of course, relation to the median 

 line as marked by the mouth and anus. — A. C. H.] 



As far as I can discover, Ehlers speaks of the blastema which 

 forms the alimentary canal as " endoderm," because it does 

 produce that structure, and naturally speaks of the remainder 

 as mesoderm, while he really has no doubt that the outer homo- 

 geneous layer is the ectoderm. In this all critics will j)robal)ly 

 agree with him, but the exact origin of this blastema has yet 

 to be demonstrated. I would, however, join issue with our 

 author on one point, and that is the origin of the tentacle-disc. 



