DEVELOPMENT. 43 



trosome. There are three theories as to the mech- 

 anism involved : 



(1) It is affirmed that the threads of the spindle 

 contract and pull each daughter loop toward its 

 centrosome. 



(2) The rays or threads may elongate and push 

 each loop toward the opposite centrosome. Accord- 

 ing to this theory there should be twice as many lines 

 or rays between the loops as there are spindle rays 

 from each centrosome, an observation recorded by 

 some investigators. 



(3) The centrosomes may be fermenting centers 

 of chemical change, and the explanation, therefore, 

 a chemical attraction or affinity. 



At the close of the metaphase the daughter loops 

 arrange themselves respectively around each cen- 

 trosome forming what is known as a diaster or 

 daughter star. The free ends of these are turned 

 away, and the curved portions of each are turned 

 toward the centrosome. 



3. Anaphase. During this period the changes 

 manifest in the prophase are reversed. The chro- 

 matin loops are gradually transformed into twisted 

 skeins of threads, called daughter skeins, which 

 ultimately produce the normal chromatin reticulum. 

 The nuclear membrane and nucleolus reappear. A 

 constriction of the cytoplasm is manifest for the first 

 time. This constriction appears in the plain passing 

 between the daughter nuclei. 



4. Telophase. In this stage the cell divides com- 

 pletely. Each daughter cell gradually assumes the 

 normal condition of the parent cell from which it had 



