CELLULAR TISSUE. 27 



Utmost consequence. The formation, propagation, and growth 

 of cells, forming, as they do, the groundwork of anatomy and 

 physiology, are* subjects which for the last few years have tasked 

 the powers of the ablest investigators. Such, however, are the in- 

 trinsic difficulties of these investigations, that the subject is still 

 involved in much obscurity, especially in regard to the formation 

 of cells ; and great differences of opinion prevail upon many other 

 essential points. At present, it is hardly possible to separate what 

 is known or reasonably well settled from what is conjectural, un- 

 proved, or untrue ; nor can the more or less conflicting views of 

 the most experienced observers be presented and explained in such 

 an elementary treatise as this.* In respect to cellular develop- 

 ment in plants, however, now that Schleiden has greatly modified 

 his views,t the highest authorities, namely, Mohl, Schleiden, and 

 Nageli, have arrived at substantially similar conclusions. These, 

 in their general outlines, may be here presented. 



26. We must distinguish between the original formation of cells 

 and their multiplication. We must also distinguish between the 

 young, vitally active cell, and the completed cell, no longer capa- 

 ble of multiplication or of having new cells formed within it. 



27. Formation Of Cells, Cells originate within other cells, or at 

 least within living tissues. J They are formed from organizable 



* The best authorities for the student to consult upon the subject are, 



1. The memoirs of Mohl in the Linncca, the Botanische Zeitung, &c., the most 

 important of which are translated in the Jlnnales des Sciences Naturelles, the 

 Annals and Magazine of Natural History, and in Taylor's Scientific Memoirs. 



2. Those of Nageli in the Zeitschrift fiir Wissensch, Botanik, whose principal 

 memoir has been translated by Henfrey for the Ray Society. 3. Schleiden's 

 Principles of Scientific Botany, translated into English by Dr. Lankester. 4. 

 Lindley's Introduction to Botany, 4th edition. 5. Henfrey's Outlines of Struc- 

 tural and Physiological Botany ; a compendious work, of which the chapters 

 on elementary structure, and all of this author's writings upon the subject, 

 are especially excellent. 



t Grundttze der Wissenschaftl. Botanik, ed. 3, reproduced in the Appendix 

 to the English translation, cited above. 



\ The Yeast-plant, developed in fermenting fluids, if that be a true vegeta- 

 tion, is an exception to the rule. According to Schleiden, this is a case of 

 "the formation of cells without the influence of another cell previously exist- 

 ing." The material has of course been elaborated in former vegetable cells ; 

 and, according to Karsten, the ferment-cells, with which the development 

 commences, already exist in the juice of the fruit, and pass through the filter 

 into the solution ; which makes this a case of cell-multiplication, rather than 

 of cell-formation. 



