MUSEUM OF COMPAKATIVE ZOOLOGY. 101 



gelatinous balls iu its cells as to be incapacitated for the work of build- 

 ing organs. In Plumatella the outer layer of the body wall, which is 

 derived, as Braem has shown, from the neck of the older polypide, re- 

 tains for a long time its embryonic condition, so that its deeper cells can 

 and do go to form the inner layer of the polypide bud. 



On Nussbaum's hypothesis we can best understand why in Paludicella 

 the Anlagen of the lateral branches exist from the beginning as cuboidal 

 cells, quite different from those of the rest of the body wall ; we can 

 understand why the cell layers of the margins of the stock, the tips of 

 branches, and the ends of stolons from which buds arise, are thicker 

 and more rapidly dividing than the rest of the body wall (cf. Figs. 14, 

 71, 73, 75) ; and we can also understand why the regenerating buds 

 always arise from the region of the neck of the degenerated polypide, 

 — the same region from which that degenerate polypide had arisen by 

 budding. 



There is no doubt, however, that at times buds do arise from tissue 

 which, as Seeliger says, has lost its cuboidal nature only to regain it. 

 From such tissue apparently the polypide of Figure 79 has arisen; 

 from such tissue certainly, as Seeliger says, do regenerating polypides 

 arise. But is the process by which cuboidal cells become a pavement 

 epithelium one of so fundamental differentiation that, in accordance 

 with Nussbaum's doctrine, we should not expect, under favorable condi- 

 tions, to see these cells regain their cuboidal form ] No doubt we have 

 many other cases in the animal kingdom in which flat epithelial cells 

 regain their cuboidal form. Thus, for instance, among the Bryozoa, 

 Oka ('90, p. 132) has shown how the flat cells of the outer layer of the 

 statoblast begin to thicken again at the return of warmth, and at the 

 beginning of the active assimilative processes, not only at the pole from 

 which the primary polypide is to arise, but also opposite to this. 



Many facts indicate that cells may become flattened epithelia, and yet 

 not lose their embryonic character. Maas ('90, pp. 541-544) has re- 

 cently shown step by step how the columnar ectoderm of the fresh water" 

 sponge is forced, on account of the great increase in surface which it is 

 called upon quickly to cover, to become broad and flat. It finally gives 

 rise to an epithelium so flat that its existence was long overlooked, and 

 has been denied by so competent an observer as Goette ; and yet in its 

 flattened condition it possesses to a remarkable degree the capacity of 

 sending out pseudopodia-like processes, a condition indicative much less 

 of a high degree of differentiation or specialization, than of an unspecial- 

 ized, primitive or embryonic condition. 



