22 GENERAL SKETCH OF THE CELL 



Kblliker, etc.) in its original narrower sense (equivalent to Stras- 

 burger's cytoplasm}, while perhaps the majority of writers have 

 accepted the terminology of Strasburger and Flemming. On the 

 whole, the terms cytoplasm and karyoplasm seem too useful to be 

 rejected, and, without attaching too much importance to them, they 

 will be employed throughout the present work. It must not, how- 

 ever, be supposed that either of the words denotes a single homo- 

 geneous substance ; for, as will soon appear, both cytoplasm and 

 karyoplasm consist of several distinct elements. 



The nucleus is usually bounded by a definite membrane, and often 

 appears to be a perfectly distinct vesicular body suspended in the 

 cytoplasm a conclusion sustained by the fact that it may move 

 actively through the latter, as often occurs in both vegetable and 

 animal cells. Careful study of the nucleus during all its phases gives, 

 however, reason to believe that its structural basis is similar to that 

 of the cell-body ; and that during the course of cell-division, when 

 the nuclear membrane usually disappears, cytoplasm and karyoplasm 

 come into direct continuity. Even in the resting cell there is good 

 evidence that both the intranuclear and the extranuclear material may 

 be structurally continuous with the nuclear membrane 1 and among the 

 Protozoa there are forms (some of the flagellates) in which no nuclear 

 membrane can at any period be seen. For these and other reasons 

 the terms "nucleus" and "cell-body" should probably be regarded as 

 only topographical expressions denoting t^vo differentiated areas in a 

 common structural basis. The terms karyoplasm and cytoplasm possess, 

 however, a specific significance owing to the fact that there is on 

 the whole a definite chemical contrast between the nuclear substance 

 and that of the cell-body, the former being characterized by the 

 abundance of a substance rich in phosphorus known as nuclein, while 

 the latter contains no true nuclein and is especially rich in albuminous 

 substances such as nucleo-albumins, albumins, globulins, and the like, 

 which contain little or no phosphorus. 



Both morphologically and physiologically the differentiation of the 

 active cell-substance into nucleus and cell-body must be regarded as a 

 fundamental character of the cell because of its universal, or all but 

 universal, occurrence, and because there is reason to believe that it is 

 in some manner an expression of the dual aspect of the fundamental 

 process of metabolism, constructive and destructive, that lies at the 

 basis of cell life. The view has been widely held that a third essen- 

 tial element is the centrosome, discovered by Flemming and Van 

 Beneden in 1875-76, and since shown to exist in a large number of 

 other cells (Figs. 7^ 8). This is an extremely minute body which 



1 Conklin ('97, i), Obst ('99), and some others have described a direct continuity in the 

 resting cell between the intranuclear and extranuclear meshworks. 



