KARYOKINESIS. 55 



in size to the quantity of cytoplasm in which they lie (figs. 16, 34, 73, etc.). We 

 have already seen that the nucleus is always proportional in size to the cytoplasm in 

 which it lies, and we are also compelled to conclude that the size of the centrosome 

 and sphere depends ultimately upon the quantity of cytoplasm. This must be taken 

 to indicate that both centrosome and* sphere receive substance from the cytoplasm 

 during their period of growth, and on the other hand it can plainly be seen that 

 the remnants of the old centrosomes and spheres are slowly transformed into cyto- 

 plasm or cell membrane after the new centers have moved out of them. There is, 

 therefore, an interchange of substance between cytoplasm and centrosome, wholly 

 similar to that between cytoplasm and nucleus (see p. 53). 



(c) . Relation of Centrosome to Nucleus. — In certain cleavages the centrosomes, 

 especially during the resting period, are very large and conspicuous, e. g., in the 

 pause preceeding the third and fourth cleavages (figs. 69, 74), they are fully six 

 (i in diameter. They contain a reticulum of material which stains blue or black 

 with hematoxylin, and on the whole they present an appearance remarkably like 

 nuclei. In no case, however, have I seen any evidence that the centrosomes are 

 directly derived from the nuclei, though this may possibly be the case in the origin 

 of the centrosomes of the first maturation division ; on the other hand they may, 

 as indicated above, receive substance which escapes in a dissolved condition from 

 the nucleus during every mitosis. 



Whatever the ultimate origin or phylogenetic relationships of the centrosome 

 may be, there is a remarkable parallelism between it and the nucleus, as the follow- 

 ing statements will show : 



1. Both begin their developmental cycle as small granules, the central cor- 

 puscle in the case of the centrosome, the chromosomes in the case of the nucleus. 



2. Both grow enormously by the absorption of surrounding substances and 

 become vesicular; in the cleavage of the egg the vesicular condition is followed by 

 a reticular condition in both. 



3. Both undergo radical changes in their staining qualities during this 

 enlargement, passing from a condition in which they are uniformly chromatic to 

 one in which they are almost entirely plasmatic in reaction ; finally, they again 

 become largely chromatic, so that the centrosomes in the resting stages of certain 

 cells look like small nuclei filled with a chromatic reticulum {cf. figs. 69, 73, 74, 75). 



4. When they have reached their greatest volume both are proportional in 

 size to the size of the cell-body in which they are found ; this probably indicates that 

 the substances absorbed by both in their growth are derived from the cytoplasm. 



5. In both, the daughter structures (centrosomes or nuclei) are but a fraction 

 of the mother organ, the remainder of the latter passing sooner or later into the 

 cytoplasm. 



The centrosome thus repeats the history of the nucleus ; at one period it takes 

 up substances from the cytoplasm ; when it has reached its greatest size the new 

 centrosomes and central spindle arise within the mother centrosome from a part of 



