Structure of Rhizopod Shells. 309 



appositional growth ; other changes, on the contrary, are only 

 to be explained by the disappearance of previously existing 

 parts 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 h priori 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 ])hylogeny. 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 



