THE MICROSCOPE IN BIOLOGY. 119 



cle. But it has since been shown that the appearance of 

 the primordial utricle is caused by the protoplasm or bio- 

 plasm lying in apposition with the inner surface of the 

 cell-wall. In the cryptogamia, cells are known to occur 

 in which no nucleus is visible. Max Schultze and Hackel 

 have also discovered non-nucleated forms of animal life. 

 The idea of nucleus and cell-wall as essential to a cell is 

 therefore abandoned. Nuclei are regarded as new centres 

 of living matter, or minute particles of such matter capa- 

 ble of independent existence. Some of these masses are 

 so small as to be barely visible with the one-fiftieth objec- 

 tive under a magnifying power of five thousand diameters. 



2. The structure and formation of a simple cell may be 

 illustrated by Plate V, Figs. 85 to 89, after Beale.* The 

 earliest condition of such a living particle is shown in 

 Plate V, Fig. 85. If the external membrane of a fully 

 developed spore or any of the growing branches (Plate V, 

 Figs. 86 to 89) be ruptured, such particles would be set 

 free in vast numbers. 



The surface of such a particle becomes altered by con- 

 tact with external agencies. A thin layer of the external 

 surface is changed into a soft membrane or cell-wall, 

 through which pabulum passes and undergoes conversion 

 into living matter, which thus increases. The increase 

 of size is not owing to the addition of new matter upon 

 the external surface, but to the access of new matter in- 

 teriorly. The thickness of the formed material depends 

 on external circumstances, as temperature, moisture, etc. 

 If these be unfavorable to the access of pabulum, layer 

 after layer of living matter will die or be deposited, as in 

 Plate Y, Figs. 87 and 88. If such a cell be exposed to 

 circumstances favorable to growth, the accession of fresh 

 pabulum will cause portions of living matter to make 



* Physiological Anatomy and Physiology of Man, by Drs. Todd, Bow- 

 man, and Beale. New edition. 



