Structure of Rhizopod Shells. 309 
appositional growth; other changes, on the contrary, are only 
to be explained by the disappearance of previously existing 
arts of the skeleton; while, finally, certain alterations are 
intelligible only by flexion of the skeletal parts involved in 
them. If we now take into consideration that the hard parts 
of the Rhizopoda consist of rigid mineral material, it is clear 
that ontogenetic developmental processes are possible only in 
the first mentioned way by the addition of new material. It 
is true that a process of resorption has already been repeatedly 
assumed to take place in the shells of Thalamophora, and 
such a process might really be conceivable, perhaps by local 
production of acid by the soft body; but this appears so pro- 
blematical that we cannot deal with this factor until its exist- 
ence has, at least once, been demonstrated with certainty. 
In the case of the siliceous skeletons of the Radiolaria a pro- 
cess of resorption is to be rejected & priort upon easily intel- 
ligible grounds. So also, of course, a flexion of rigid calca- 
reous and siliceous parts is impossible. Hence it appears 
that the ontogenetic development of the hard parts of the 
Rhizopoda can take place only by appositional growth, and 
all structures which cannot be explained thereby must be 
ascribed to phylogenetic development, as of course by means 
of phylogeny any conceivable alteration of form is possible. 
‘The circumstance that in the case of the hard parts when 
once secreted, subsequent re-solution or alteration by total or 
local resorption, flexion, extension, and the like is no longer 
possible, involves another exceedingly important consequence. 
As in the higher Protista, in which already we may speak of 
a true individual development, and which therefore have their 
genealogy behind them, and to which, of course, the bioge- 
netic fundamental law applies as to plants and animals, so 
also the ontogeny of the skeleton of the Rizopoda furnishes 
a more or less exact reproduction of their phylogeny. But 
while, in the’ higher organisms after the completion of the 
ontogeny, the individual stages passed through during the 
latter have generally long since disappeared, in the Rhizopod 
skeleton the entire development which has been passed 
through is still completely preserved in the adult specimen. 
In order to obtain an accurate picture of the development of 
the shell, it is only necessary to examine the earlier-formed 
parts back to the youngest, therefore in shells with concentric 
growth to pass from the centre to the periphery, and in those 
with terminal growth, from the so called embryonal chamber 
along the series of chambers to the end. Therefore, as in 
the known example of the Cephalopod shell, it is very often 
possible also in the Rhizopod shell to compare directly the 
