352 



A. RICHARDS. 



Somatic tissue cells have been followed through their divisions 

 by a few investigators who have observed them either in growing 

 mesenchyme of a tadpole tail (Clarke) or in tissue cultures (Lam- 

 bert, Lambert and Hanes, Levi, and Lewis and Lewis). Clarke 

 observed that the prophase and metaphase together lasted about 

 i hour and 15 minutes, the anaphase 4 minutes, and the telophase 

 3 to 4 minutes (although complete separation of the cytoplasm 

 followed much more slowly). Lambert and Hanes found in tissue 

 culture that at 37 C. connective tissue cells of the cat divide in 

 15 to 30 minutes, while mitosis in similar cells of the rat last from 

 25 to 45 minutes. Levi studied tissue cultures from chick em- 

 bryos and arrived at the following range of variation for the 

 phases of mitosis therein : Prophase and metaphase together. 20 to 

 30 minutes, of which 8 to 13 minutes, according to further observa- 

 tions of his, were consumed by the metaphase ; anaphases usually 

 range from 3 to 7 minutes; telophases last from i to 10 minutes, 

 although the great majority were from 3 to 6 minutes in duration. 

 Lewis and Lewis made careful observations on the duration of 

 various phases of mitosis in mesenchyme cells in tissue cultures 

 and concluded that the time ranges for the various stages are as 

 follows: prophase, 30 to 60 minutes; metaphase, 2 to 10 minutes; 

 anaphase, 2 to 3 minutes; telophase, 3 to 12 minutes; and the 

 reconstruction period, 30 to 180 minutes. 



Laughlin has made the most extensive study of the duration of 

 mitotic stages in dividing root tip cells of the onion. He employed 



