THE MECHANISM OF LIFE 543 



been irremediably injured in the division, regeneration is no 

 longer possible, because the necessary molecules and atoms are 

 no longer produced and brought to the necessary place, Hence 

 it is a universal and fundamental fact that non -nucleated pieces of 

 a cell, i.e., pieces into which a fatal disturbance of metabolism has 

 come, do not regenerate lost parts, although under certain circum- 

 stances they are able to live for days. 



One phenomenon, which some years ago appeared very puzzling, 

 is approximately explained by the fact of structures in the cell- 

 protoplasm. This is the formation of very regular silicious and 

 calcareous skeletons, especially in the delicate Radiolaria, Foramini- 

 fera, and sponges. F. E. Schulze, ('87) called attention to the 

 fact that the formation of triradiate and quadriradiate spicules 



A B 



"Fio. 267. Silicious skeletons of Radiolaria. (After Haeckel.) A, Dorataspis. , Theoconus. 



(Fig. 265, //), which play so great a role in the silicious and 

 calcareous skeletons of sponges, must take place when several 

 spherical bodies are in contact with one another and a skeleto- 

 genous substance, such as calcium carbonate or silicic acid, is 

 excreted into the fine spaces between them (Fig. 265, /). Lately 

 Dreyer ('92) has extended the same idea to several special 

 examples, and has shown how various and often extremely 

 complex skeletal parts, especially in the Radiolaria, may easily 

 be traced to the excretion of skeletogenous substance in the 

 protoplasmic walls of a vacuolar layer (Fig. 266). Thus, according 

 to the form of the vacuoles, the thickness of their walls, the place 

 at which the secreted skeletal substance is deposited, and its 

 quantity, a great variety of skeletal forms must result, and are 

 actually realised in the richly varied forms of the radiolarian 



