MUSEUM OF COMPARATIVE ZOOLOGY. 65 



layers of the body wall (i., ex.). Owing to the reagent, the body wall is 

 shrunken from its contact with the operculum (op.). 



If one inquires what has been the histological conditions of this region 

 antecedent to this, stage, one must look to younger adjacent and mar- 

 ginal zooecia, since they reproduce these conditions. I will again call 

 attention to Figure 88, which represents a cross section of the body wall 

 through the region of attachment of the kamptoderm of a young pol- 

 ypide of about the stage of Figure 83. This, then, represents the neck 

 of the polypide, and it is from about this region that the operculum and 

 finally the regenerating polypides will arise. The cells are columnar, 

 and stain deeply about the nuclei, and both cell layers are well devel- 

 oped. Elsewhere in this same individual the body wall is composed of 

 smaller, flatter cells, and two layers are not easily distinguished. The 

 region of the future operculuip possesses at an early stage some of the 

 largest, most columnar cells of the body wall. The cells of this region 

 do not, however, retain their peculiarly large size throughout life, but 

 in the adult we find the same region occupied by a flat epithelium, 

 nearly as thin as the epithelium shown in Figure 90. Meanwhile 

 the epithelium of the rest of the body wall has become still more 

 attenuated. The difi"erence between the body wall of the operculum 

 and that of adjacent regions is best shown by the greater abun- 

 dance of nuclei under the opercular region when the stained stock is 

 looked at m toto from the roof (Plate VIII. Fig. 71). The regions of 

 the future opercula are seen, in young zooecia (Fig. 71, 4, 6), to be 

 patches of densely packed nuclei. The opercula of older zooecia show a 

 slight preponderance of nuclei, and thus indicate more numerous cells. 

 It is from such a region, then, that the young regenerating polypide 

 arises. 



As in the case of the marginal polypides, so here, the lips of the 

 invagination pocket close and become fused to form the neck of the 

 polypide (Plate X. Fig. 84). The later stages of the development of 

 the regenerating polypides seem to be the same as those of the marginal 

 buds. Figures 74 and 89 ai-e, indeed, regenerating polypides. I cannot 

 find any evidence that the alimentary tract, or any part of it, is formed 

 in regenerating buds by a method differing in any essential particular 

 from that in marginal buds. 



It is well known, however, that the degenerated polypide which forms 

 a " bi'own body " in the old zooecium eventually disappears. Haddon 

 ('83, p. 519) maintains that in the developing regenerated polypide 

 " the walls of the stomach, or, more strictly, that portion of the stomach 



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