136 Life and Death, Heredity and Evolution 



Sometimes, as in the case just described, it is two nuclei of 

 the same cell that reunite. In other cases it is half nuclei 

 from two adjoining cells of the same organism; this hap- 

 pens in the alga Spirogyra (Figure 42). Or again, two 

 branches of the same plant may unite, with union of nuclei 

 (Figure 43). Or again, two cells that have been formed by 

 the division of a single cell may mate ; this happens in Para- 

 mecium aurelia at times, as well as in other infusoria, and 

 in algae. We find every possible transition, from the union 

 of two half nuclei of a single cell (Figure 41) to the fertili- 

 zation of one individual by another quite unrelated to it. 



In all these cases in which the two half nuclei that unite 

 have been developing together in a single nucleus, evidently 



Figure 43. Process of conjugation of two branches of the same 

 plant, in the mould Zygorhynchus. A to H, successive stages, leading 

 to the formation of the dark zygospore in H. After Blakeslee, 1913. 



