2 74 BIOLOGICAL LECTURES. 



Such are some of the questions that are being asked on 

 every side. But it is evident that there are certain problems 

 which must take precedence of others. Such, for example, is 

 the question relating to the mode of its development in the 

 cell. And it is along this line that the inquiries have of late 

 been most active. It is to this subject that I propose to in- 

 vite your attention. 



So far as I have been able to gather, no less than seven 

 different hypotheses have been proposed in regard to the nature 

 and origin of this structure. This is neither the place nor the 

 occasion, however, to enter into any critical examination of 

 technical details. Suffice it to say, that these divers hypoth- 

 eses may be reduced to two fundamental forms, which are 

 mutually exclusive of each other. 



According to the one view (i), the centrosome is a perma- 

 nent or ultimate organ of the cell, an organ siii generis, and 

 coexistent with other ultimate organs of the cell, as the nucleus 

 and the cytoplasm. 



According to the other view (2), the centrosome is a derivative 

 structure, arising by the modification of some preexisting element 

 in the cell, as the chromosome, "nucleolus," or the cytoplasm. 



I repeat, that these two views are mutually exclusive ; be- 

 cause, if the centrosome is considered to be a permanent organ 

 of the cell, that is to say, if it always originates from a pre- 

 existing centrosome, obviously it cannot be considered as a 

 derivative from another structure. If, on the other hand, it 

 can be maintained that it is a derived structure, it can neither 

 be called a permanent nor an ultimate organ of the cell. 



Thus, from the nature of the case, no middle ground is pos- 

 sible. It is rare that an investigator is confronted with alter- 

 natives so sharply contrasted; nor does he often meet with an 

 issue that can be brought to so sharp a focus. For, if it could 

 be shown by the examination of cell-structure that there exists 

 any element which has a close affinity to, or identity with, 

 the centrosome, its claim as a unique organ must fall to the 

 ground. Only utter failure to identify the centrosome with 

 any other element in the cell could justify the adoption of the 

 theory that it is a unique organ. 



