6oo Church. — The Structure of the Thallus of 
Acetabularia crenulata are undoubtedly secondary, and this 
leads to the suggestion that the other septum below the 
coronae may possibly represent the formation of a special 
septum for the purpose of smoothly detaching the reproductive 
rays, at some earlier stage in the phylogeny. 
The fifth type of leaf closes the series ; other genera at this 
point, owing possibly to the greater capacity for protection in 
the cavities of the thallus, follow the plan of the simpler genera 
of the Dasycladus group, and originate the gametangium as 
a new outgrowth, either laterally as in Bornetella , or in the 
angle of ramification of the first dichotomy ( Neomeris ) or 
polytomy ( Cympolia ). This again may be regarded as a transi- 
tion to a ‘ paraphysial type,’ in which protection is afforded 
to the reproductive organs while in the early stages of 
development. At any rate, this is probably the function 
of the cortical layer in Neomeris ; thus the gametangia are 
developed later, and when they are ready for dispersal, the 
calcified layer, either from contraction of the older internodes, 
or decalcification of its now non-assimilating members, becomes 
worn away and exposes them freely. 
Finally, N. dumetosa suggests the lines along which differ- 
entiation has taken place in N. Kelleri and Cympolia. The 
calcification en masse of the latter is due, as Solms points out, 
to the fusion of the mucilaginous layers in the internal 
cavities of the thallus ; while similar calcification of groups 
of adjacent gametangia in N. Kelleri into block-masses, so 
excludes light from the basal segments that they soon cease 
accumulating calcium carbonate. The tendency of A. dumetosa 
to reversion to a lower order of leaf is carried still further in 
Cympolia : cases in which N. dumetosa tended to revert after 
a dozen whorls or so have already been described ; C. barbata , 
according to Cramer, reverts after 4-16 whorls of Stage V 
right back to 5-10 whorls of Stage II. Such extensive 
reversion was observed in one specimen only of N. dumetosa ; 
the reversion and dichotomous branching at this stage, noticed 
as exceptions in the case of N. dumetosa , thus becoming the 
normal condition in Cympolia barbata. 
