64 MANUAL OF HISTOLOGY. 



including within it a peculiar structure. These parts require special 

 names. The soft substance mentioned is known as the cell-substance or 

 cell-body (b b), the central structure enclosed within it as the nucleus 

 (c c), and a minute dot-like particle situated within the latter again as the 

 nucleolus (d d). 



The external boundary of the cell (a a) is in certain cases formed by 

 the soft mass alluded to, or more frequently by a somewhat hardened 

 stratum, the enveloping or cortical layer, or, finally, by a distinct indepen- 

 dent pellicle separable from the cell-body, and known as the cell-mem- 

 brane. 



In regard to the latter, the views entertained respecting the animal cell 

 have latterly undergone considerable change through the results of recent 

 investigation. The presence of a special membrane was formerly con- 

 sidered necessary (Schwann) to the conception of a true cell; but the 

 frequent absence of this envelope and its relatively small physiological 

 significance has been since recognised (Schultze, Briicke, Beale). 



]3ut although its anatomical characters offer us the first and most 

 important points in the definition of the cell, its physiological properties 

 cannot be overlooked. By these the cell is constituted a living structure, 

 endowed with special energies and the peculiarities of active vitality; with 

 the power of absorption of matter, of transforming the same, and of excre- 

 tion ; with the capability of growth, of change of form, and of cohesion or 

 fusion with similar organisms. The cell possesses further, undeniably 

 although there may be a variety of opinion as regards the extent of these 

 powers in individual cases the capability of vital motion, as well as of 

 proliferation, or the generation of a progeny. The cell, we repeat it, is 

 the earliest physiological unit, the first physiological apparatus : it has 

 been called an "elementary organism" with propriety. 



One of the most important facts established by recent scientific 

 investigation is, that that mass from which the bodies of all the higher 

 animals take their origin, namely, the ovum, has entirely the nature of a 

 cell, so that, consequently, each such animal body, be it ever so complex 

 in constitution, once consisted of one single cell. While in this respect 

 the- latter must be regarded as the starting-point of animal life, naturalists 

 again have brought to light creatures of such simple organisation that 

 their whole body is formed of nothing more than one independent cell, and 

 whose whole existence is included within the narrow circle of cell-activity. 

 Among such may be reckoned those animals known as gregarines. 

 Finally, single-celled plants have been discovered by botanists, as single- 

 celled animals by anatomists : and even still more rudimentary organisms 

 have been met with. 



REMARKS. Compare the work of this author, " Mikroscopische Untersuchungen 

 iiber die Ubereinstimmung in der Struktur and dem Wachsthum der Thiere und 

 Pflanzen ;" also L. Beale, "The Structure of the Simple Tissues of the Human Body," 

 Lond. 1861. But is the cell the simplest "elementary organism,'' i.e., the simplest 

 structure which can meet all the requirements of the lowest grade of life ? This 

 question may be negatived. An excellent observer, E. HdcTcel (Generelle Morphologic, 

 Band 1, s. 269, Berlin, 1866 ; and Biologische Studien, Heit 1, s. 77, Leipzig, 

 1870), has shown that a particle of protoplasm, or "cytode," as he terms it, suffices 

 for this. It is only subsequently, after the generation of a nucleus that the whole 

 becomes a cell. It is, nevertheless, a deeply significant fact, that the building 

 stones of the bodies of higher animals are never represented by "cytodes," but 

 always by cells. 



