256 THE MICROSCOPE. 



Mirbel had shown that all the different forms of vege- 

 table tissue are developed from cells which enter into the 

 formation of the embryo plant. Schleiden followed Mir- 

 bel in tracing out the development of the tissues of the 

 fully formed plant from the nucleated cells composing 

 the embryo ; and he also studied the mode of formation 

 of the nucleated cell itself. On this point Schleiden 

 came to the conclusion that the nucleus is the germ of the 

 plant-cell, hence he named it the " cytoblast." Miiller 

 subsequently contended that the spinal chord is com- 

 posed of cells, resembling vegetable cells ; Schwann dis- 

 covered a nucleus in these cells, and observed that 

 the various forms of cells in animal structures is 

 similar in every respect to those of plants. From 

 his investigations he was led to the philosophical 

 generalization, that the tissues of the animal body 

 and those of plants were formed from cells. The 

 various tissues, although formed from cells in 

 different stages of their development, and not neces- 

 sarily the formative element of all cells in their fully 

 formed stage, for cells, when fully formed, in some cases 

 do not undergo further development ; for example, in 

 the parenchyma of glands when they break up, and 

 are resolved into the secretive matter. 



The prevailing opinion now is and of which neither 

 Schleiden nor Schwann appear to have had a true 

 notion that nuclei and cells are propagated by the sub- 

 division of pre-existing nuclei and cells. As to the 

 particular endowments and potentialities of the dif- 

 ferent kinds of cells whereby each is developed and 

 converted into its own special tissue, and indifferently 

 into any kind of tissue, I must refrain from discussing, 

 as it would lead me into a region of speculation. 



" If nature," writes Humboldt, " had endowed us 

 with microscopic powers of sight, and if the integu- 

 ments of plants were transparent, the vegetable king- 

 dom would by no means present that aspect of 

 immobility and repose under which it appears to our 

 senses." And so with regard to the instruments of 

 motion in the higher classes of creation, the muscles 

 of animals very soon disappear as we descend in the 



