54 



HISTORY OF THE HUMAN BODY 



that is still unknown. It has shown the nucleus to be a mi- 

 crocosm of extraordinary complexity, and has" opened up a 

 new world, the very existence of which has until lately re- 

 mained unsuspected. 



What seems to be the essential element of all nuclei, found 

 alike in plants and animals, is a substance which, from its 

 extreme susceptibility to staining fluids when artifically treated 

 for purposes of microscopic examination, has been designated 

 by the non-committal term of chromatin. During functional 



VII 



VIII 



FIG. 10. Diagrams representing normal mitosis. 



In I the nucleus is "resting"; the centrosome is seen by its side. In II the 

 spireme appears, which in III becomes separated into chromosomes. In IV the 

 centrosomes have become placed at opposite poles, while the chromosomes form 

 an equatorial plate midway between them. Each chromosome divides longitudinally 

 in V, and in VI and VII becomes drawn to the two opposite poles. In VIII the 

 cell divides into two. 



activity this substance is diffused throughout the nucleus in 

 little, irregular masses, but assumes the form of a continuous 

 thread or chain preparatory to a cell division, and eventually 

 becomes separated into a definite number of equal bodies, the 

 chromosomes. The number of these found in any somatic 

 cell of a given species of animal is always the same for that 

 species, but may be different in an allied species, and the num- 

 ber seems to bear no reference to the size or the degree of 

 complexity of the animal. For instance, the number four oc- 

 curs in Ascaris, the pin-worm, eight in certain nematode 



