52 G EX ERA I. SKETCH OE THE CELL 



view it was regarded turlher as the especial fertilizing element in the 

 spermatozoon, which, when introduced into the egg, endowed the 

 latter \\ ilh the power of division and development. Van Beneden's 

 and Boveri's hypothesis, highly attractive on account of its simjjlicity 

 and lucidity, is supported by many facts, and undoubtedly contains an 

 element of truth ; yet recent researches have cast grave doubt upon 

 its generality, and necessitate a suspension of judgment upon the 

 entire matter. Many of the most competent recent workers on the 

 cytology of higher plants have been unable to find centrosomes, 

 whether in the resting-cells, in the apparatus of cell-division, or dur- 

 ing the process of fertilization, notwithstanding the fact that undoubted 

 centrosomes occur in some of the lower plants. Among zoologists, 

 too, an increasing number of recent investigators, armed with the 

 best technique, have maintained the total disappearance of the cen- 

 trosome at the close of cell-division or during the process of fertili- 

 zation, agreeing that in such cases the centrosome is subsequently 

 formed dc not'o. Experimental researches, also, have given strong 

 ground for the conclusion that cells placed under abnormal chemical 

 conditions may form new centrosomes (p. 306). If these strongly 

 supported results be well founded. Van Beneden's hypothesis must 

 be abandoned in favour of the view that the centrosome is but a sub- 

 ordinate part of the general apparatus of mitosis, and one which may 

 be entirely dispensed with. Thus regarded, the centrosome would 

 lose somewhat of the significance first attributed to it, though still 

 remaining a highly interesting object for further research.^ 



F. Other Organs 



The cell-substance is often differentiated into other more or less 

 definite structures, sometimes of a transitory character, sometimes 

 showing a constancy and morphological persistency comparable with 

 that of the nucleus and centrosome. From a general point of view 

 the most interesting of these are the bodies known 3.s plasiids or proto- 

 plasts {¥\g. 6), which, like the nucleus and centrosome, are capable of 

 grow^th and division, and may thus be handed on from cell to cell. 

 The most im.portant of these are the cJironiatopJioirs or cJirouwplastids, 

 which are especially characteristic of plants, though they occur in 

 some animals as well. These are definite bodies, varying greatly 

 in form and size, which possess the power of growth and division, and 

 have in some cases been traced back to minute colourless plastids or 



^ ^f- PP- III' 304- Eisen ('97) asserts that in the blood of a salamander, Batraclioseps, 

 the attraction-sphere (" archosome ") containing the centrosomes may separate from the 

 remainder of the cell (nucleated red corpuscles) to form an independent form of blood- 

 corpuscle or " plasmocyte," which leads an active life in the blood. 



