286 BIOLOGICAL LECTURES. 



There is one important fact which, at first sight, appears to 

 militate against the view that the centrosome is cytoplasmic 

 in its origin. I refer to the observation that the centrosome 

 originates inside the nucleus in some cells. Of course, as I 

 have stated elsewhere, when the centrosome originates inside 

 the nuclear membrane, it may be said to be derived from the 

 "nucleus"; when it originates outside of the nuclear mem- 

 brane, it may be said to be "cytoplasmic" in origin. Such a 

 distinction is a purely nominal one, however, from my stand- 

 point, and I believe the general statement that all centrosomes 

 are cytoplasmic in their origin is fundamentally a correct one. 

 Confusion only arises when we do not keep in mind the fact 

 that the cytoplasmic net-work, in the substance of which the 

 microsome and centrosome arise, exists on both sides of the 

 nuclear membrane, and the structure known as "nucleus" con- 

 tains, besides the chromosomes, a certain quantity of the cyto- 

 plasmic substance in it. Such intranuclear cytoplasm passes 

 under the name of linin. The mere fact, therefore, that the 

 centrosome originates inside the nucleus does not show that it 

 is derived from the chromosome, which, though essential, is 

 but one of the nuclear constituents. 



As to the often repeated statement that the centrosome is 

 derived from the "nucleolus," it will become more valuable 

 when the nature and origin of such "nucleolus" are more 

 clearly given. 



It can no longer be doubted that in certain cases the centro- 

 some first assumes its visible form inside the nucleus, from 

 which it emerges into the cell-body through the nuclear mem- 

 brane. It will not be going too far when I suggest that such 

 a centrosome is probably formed by the intranuclear cytoplasm 

 in the same manner as the centrosome outside the nuclear 

 membrane. Is not some "nucleolus," which is said to give 

 rise to the centrosome in certain cases, the centrosome itself, 

 formed by the intranuclear cytoplasm also } 



But, whatever view one may take in regard to the nature of 

 the centrosome, one thing is clear, viz., that the centrosomes 

 offer a great deal of structural difference in different cells. 

 In one cell it may assume a spherical shape composed of deeply 



