D THE CELL 



as a small chamber, or cellula, in the true sense of the word. They 

 considered the membrane to be the most important and essential 

 part of the vesicle, for they thought that in consequence of its 

 chemico-physical properties it regulated the metabolism of the 

 cell. According to Schwann, the cell is an organic crystal, which is 

 formed by a kind of crystallisation process from an organic mother- 

 substance (cytoblastema) . 



The series of conceptions, which we now associate with the 

 word " cell," are, thanks to the great progress made during the 

 last fifty years, essentially different from the above. Schleiden 

 and Schwann's cell-theory has undergone a radical reform, having 

 been superseded by the Protoplasmic theory, which is especially 

 associated with the name of Max Schultze. 



The History of the Protoplasmic theory is also of supreme interest. 

 Even Schleiden observed in the plant cell, in addition to the cell 

 sap, a delicate transparent substance containing small granules ; 

 this substance he called plant slime. In the year 1846 Mohl 

 (I. 18) called it Protoplasm, a name which has since become so 

 significant, and which before had been used by Purkinje (I. 24) 

 for the substance of which the youngest animal embryos are 

 formed. Further, he presented a new picture of the living 

 appearances of plant protoplasm ; he discovered that it completely 

 filled up the interior of young plant cells, and that in larger and 

 older cells it absorbed fluid, which collected into droplets or 

 vacuoles. Finally, Mohl established the fact that protoplasm, as 

 had been already stated by Schleiden about the plant slime, shows 

 strikingly peculiar movements ; these were first discovered in the 

 year 1772 by Bonaventura Corti, and later in 1807 by C. L. 

 Treviranus, and were described as " the circulatory movements of 

 the cell-sap." 



By degrees further discoveries were made, which added to the 

 importance attached to these protoplasmic contents of the cell. 

 In the lowest algae, as was observed by Cohn (I. 7) and others, 

 the protoplasm draws itself away from the cell membrane at the 

 time of reproduction, and forms a naked oval body, the swarm- 

 spore, which lies freely in the cell cavity ; this swarm-spore soon 

 breaks down the membrane at one spot, after which it creeps out 

 through the opening, and swims about in the water by means of 

 its cilia, like an independent organism ; but it has no cell mem- 

 brane. 



Similar facts were discovered through the study of the animal 



