544 GENERAL PHYSIOLOGY 



skeleton. Thus the fact, which previously seemed so wonderful,, 

 that the simple protoplasmic masses of rhizopod cells, while con- 

 tinually engaged in streaming and forming pseudopodia, are able 

 to construct such astonishingly regular, complicated, and delicate 

 skeletons, is at once understood from the fact that the protoplasm 

 of these cells possesses in a certain body-zone a vacuolar or honey- 

 comb structure. According to the form, the position and the 

 extent of this vacuolar layer and its vacuoles, the effusions, which 

 result from the excretion of skeletal substance between the 

 vacuoles and form the skeleton, must vary extraordinarily (Fig. 

 267). Doubtless a role similar to this of the vacuolar structure 

 of the protoplasm in the formation of many radiolarian skeletons 

 is played by the structure of protoplasm, as well as by the form 

 and the mutual pressure of the individual cells, in the formation 

 of the skeleton in other organisms. 



d. The Mechanics of Hereditary Transmission 



There finally remains a brief examination of the mechanics of 

 hereditary transmission. The conditions of hereditary trans- 

 mission are simplest in the lowest unicellular organisms ; for 

 example, in Amoeba, apart from an increase in the size of the 

 body, no distinct development is observable. Here, where the 

 reproduction of the organism takes place simply by the division 

 of the cell into two halves, the process of the transmission of 

 all the characteristics of the mother-cell to the two daughter-cells 

 is at once comprehensible. The living substance of the mother- 

 cell with its characteristic metabolism and its peculiar vital 

 phenomena, continues to live independently in the daughter-cells ;. 

 it is no wonder, therefore, that the separate pieces, when living 

 under the same external conditions, possess exactly the same 

 characteristics that the undivided cell possessed. But this-' 

 simplest case of inheritance exhibits very clearly the essential 

 factors of the phenomenon, just as all vital phenomena in general 

 are to be seen and understood most clearly where they appear in* 

 their simplest form, i.e., in the simplest cells. It is seen here 

 that the transference of the characteristics of the ancestors to 

 the descendants, takes place by the transference of substance 

 which possesses the characteristics of the ancestors. In order 

 that this substance may possess all the characteristics of the 

 latter, it must be a complete cell with all the essential cell- 

 constituents. The characteristic peculiarities of the mother-cell 

 are the expression of its metabolism. If, therefore, the pecu- 

 liarities of the mother-cell are to be transmitted to the daughter- 

 cells, its whole metabolism must be transmitted. But this is possible 

 only when a certain quantity of all the essential constituents, i.e., 

 of the protoplasm and nucleus of the mother-cell, passes over to- 



