Cell Division and Heredity 67 



from each other laterally but will remain linked 

 together. Since the spireme threads or prophase 

 chromosomes become separated from each other 

 completely after longitudinal splitting, they could 

 not have been constructed in this manner, but 

 must have had a construction similar to that 

 shown in Fig. 10 wherein the thread itself is 

 twisted in a direction opposite to the direction in 

 which the coil is wound. Kegardless of whether 

 this structure remains in the form of a coil or is 

 drawn out straight, the two halves can be com- 

 pletely separated from each other laterally. 



In the non-sexual unicellular organisms the life 

 cycle is complete after each cell division, but in 

 the higher forms of life there is not only the life 

 cycle of each individual cell, but also the larger 

 life cycle of the organism as a whole. It is a sig- 

 nificant fact that this larger life cycle with its ex- 

 tensive diversification of tissues occurs only in 

 those organisms which possess sex. 



It is usually assumed that sex is an adaptation 

 by nature for the purpose of promoting certain 

 phases of biological economy or for increasing the 

 variability of the species. These assumptions, 

 however, are not adequately supported by facts, 

 since actual observations of the lowest organisms 

 which exhibit conjugation, as for example the 



