MUSEUM OF COMPARATIVE ZOÖLOGY. 101 
gelatinous balls in 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 Secliger 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 favo rable 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. 182) 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 beeome flattened epithelia, and yet 
not lose their embryonic character. Maas (790, pp. 541-54 4) 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, 
