DEVELOPMENT. 41 



division, and refers to the changes manifest in the 

 nucleus during such division. The process may be 

 divided into four phases 



i. Prophase. The first manifestation of mitosis 

 appears in the centrosome. This little body is, 

 therefore, looked upon as the dynamic center of the 

 cell. The centrosome. divides and each half passes 

 to opposite poles of the nucleus. The chromatin 

 network, which really consists of chromatin granules, 

 is transformed into a skein of threads known as a 

 spirem or mother skein. These threads break up into 

 a definite number of segments known as chromo- 

 somes. The chromosomes are even in number, and, 

 for a given species, a constant number is always 

 present. In the human cell there are sixteen chro- 

 mosomes, according to some, twenty-four. To- 

 ward the close of this stage, each chromosome 

 forms a loop and is finally arranged symmetrically 

 around the equator of the nucleus, with the free ends 

 of each loop turned away from the center. This 

 symmetrical figure is known as monaster. During 

 this stage the nuclear membrane usually disappears. 

 The nucleolus also vanishes, just how is not clear. 

 The net knots disappear with the formation of 

 chromatin loops. 



During these changes the achromatic substance 

 linin and nuclear sap has formed a central spindle 

 with each apex directed toward a centrosome. This 

 spindle consists of fine threads or lines directed 

 toward the centrosomes. A like radiation becomes 

 manifest in the cytoplasm, each line being directed 

 toward, or from, one or the other centrosome. This 



