GENERAL ANATOMY. 6T 



nucleoli which contain, the one chromatin, the other exclusively 

 plastin (4). In tissue cells only the plastin has the form of 

 nucleoli (true or chromatin-free nucleoli, 5), while the chromatin 

 is distributed on the nuclear reticulum (chromatin reticulum). 

 Somewhat the same may occur in the Protozoa (fig. 19, 3). 



Significance of the Cell Nucleus For a long time the func- 

 tional significance of the nucleus in the cell was shrouded in 

 complete darkness, so that it began to be regarded, in comparison 

 with the protoplasm, as a thing of little importance. The evidence 

 that the nucleus plays the most prominent role in fertilization has 

 altered this conception. Then arose the view that the nucleus 

 determines the character of the cell ; that the potentiality of the 

 protoplasm is influenced by the nucleus. If from the egg a definite 

 kind of animal develop, if a cell in the animal's body assume a 

 definite histological character, we are, at the present time, inclined! 

 to ascribe this to the nucleus. From this, then, it follows farther 

 that the nucleus is also the bearer of heredity; for the transmission 

 of the parental characteristics to the children (a fact shown to us 

 by our daily experience) can only be accomplished through the 

 sexual cells of the parents, the egg and sperm cells. Again, since 

 the character of the sexual cells is determined by the nucleus, the 

 transmission in its ultimate analysis is carried on by the nucleus. 

 This idea has a further support in experiments on Protozoa. If 

 one of these unicellular animals be cut into nucleate and anucleate 

 halves, the latter sooner or later degenerates, the former persists 

 and regenerates the lost parts. Within the nucleus it is probably 

 the chromatin which controls the functions of the protoplasm and 

 is accordingly (as observations on fertilization also seem to show) 

 the bearer of heredity, while the achromatin is the seat of contrac- 

 tility, and as such plays a part in cell multiplication. 



The Centrosome. Besides the nucleus there frequently occurs 

 a special body in the protoplasm, the centrosome, which on 

 account of its small size and a behavior similar to achromatin 

 with reference to staining-fluids was long overlooked, and even 

 now its demonstration is difficult. It is apparently well distributed 

 among the Metazoa, but is absent from most Protozoa. In many 

 it appears only at certain times and then disappears. What is 

 known of it makes it probable that it is a derivative of the nucleus, 

 a part of the achromatin which has left the nucleus; in other cases 

 possibly a second nucleus which by degeneration has lost the 

 chromatin and retained only the active nuclear substance, the 

 achromatin. In its function the centrosome is a specific organ of 



