OP ARTS AND SCIENCES. 279 



polypides, one of which is slightly older than the other, arise so that 

 their anal surfaces are turned towards each other. 



The c(clomic epithelium arises by a sort of ingression, as observed 

 by Korotiieff, and is to be regarded, probably, as mesoderm plus ento- 

 derm, the entoderm being reduced to a still more rudimentary condition 

 than in Gymuolicmata. 



In I^lumatella, the mner layer of the primary polypide arises near 

 the pole of ingression directly from the outer layer of the two-layered 

 sac, and not from a mass which has already lost its connection with 

 the outer layer, as in Cristatella. The second polypide arises at some 

 distance from and wholly independently of the first, and, like it, by an 

 invagination of the body-wall near the pole of ingression. The re- 

 mainder of the polypides are derived from the indifferent cells about 

 and in the neck of these two primary ones. 



The method of origin of the primary polypides is fundamentally the 

 same in both genera, but the conditions in Plumatella are to be re- 

 garded as the more primitive. In both, the inner layer of the poly- 

 pide is derived from the neutral region of the outer larval layer 

 whence the inner larval layer has arisen. Possibly this region should 

 be regarded as neither ectoderm nor entoderm, but as still indifferent, 

 and capable of giving rise to either. 



The origin of the primary bud in Gymnolasmata also is probably 

 to be referred to the pole of ingression or invagination, but owing to 

 greater difficulties of orientation this cannot be determined so easily as 

 in Phylactola^mata. 



My studies on and drawings of Paludiceila were already nearly 

 completed when I first saw Braem's " Untersuchungen iiber die Bryo- 

 zoen des siissen Wassers," in Bibliotheca Zoologica, Heft VI., 1891. 

 With great keenness, he has been able, even by the study of the living 

 animal, to correct some errors of previous authors, and he has antici- 

 pated some of my observations. 



Each young polypide arises in the adult colony independently of 

 any older polypide, from a mass of embryonic, rapidly dividing tissue 

 at the tip of the branch, and some of this tissue is left behind as the 

 tip moves forward. Typically, three masses of such tissue are left 

 behind, at intervals corresponding to the joints of the branch. Of 

 these three masses, one, the median, gives rise directly to the youngest 

 polypide of the ancestral branch. The other two lie about 90° to the 

 right and left of the median bud, and remain in a quiescent state until 

 the median bud has attained a considerable size. The cells of these 

 lateral masses are always distinguishable from those of the adjacent 



