436 
l'\ A. POTTS. 
still preserved iu the proliferating lip and its neighbonrliood, 
but in tlie parts opposite more fully developed stolons the 
interior of the cushion is occupied by scaiity and vacuolated 
tissue, consisting only of the basis of fibres, connective-tissue 
and muscular, which can be noticed in the younger parts. 
But the cellular elements are well-nigh absent, and it must be 
supposed that they migrated into the stolons while they were 
attaining their great development. The ectodermal epithelium, 
too, is much vacuolated and contains sparsely scattered nuclei. 
Text-fig. 6, representing T. crosslandi (2), shows a still 
more advanced stage. That part of the cushion where pro- 
liferation is still going on and the cells of the mesoblast still 
fill the interior is reduced to small proportions. The l est of 
the cushion, extending far dorsalward, has thinned out con- 
siderably, being absorbed in long processes which support 
small groups of stolons. It is quite evident that when pro- 
liferation has finally ceased and the stolons have all broken 
away, the cushion disappears and the body-wall is reduced to 
its normal proportions. 
The C(Elomic Cavity during Proliferation. 
During the early stages of proliferation the coelom does not 
extend into the cushion, although the leucocytes probably 
invade the tissue in great numbers and play a most impor- 
tant part. In fact the cavity of the coelom is encroached upon 
and almost obliterated by the proliferation of mesoblast, which 
is not confined to the ventral cushion but also goes on dorsally 
(see T. crosslandi. Text-fig. 2). 
In T. gemmipara, at a later stage (Text-fig. 7), a diverti- 
culum of the coelomic cavity does invade the cushion in the 
neighbourhood of the proliferating lip. The coelomic epithe- 
lium here appears to be specially thickened, but I do not know 
if the cells which compose it migrate into and reinforce the 
mesoblast of the cushion. 
In T. crosslandi at a still later stage (Text-fig. 6) the 
coelom is prolonged into the lateral outgrowths of the cushion 
from which the stolons depend. 
