182 Life and Death, Heredity and Evolution 



Figure 49. The chromosomes and the divisions preparatory to mating 

 in Paramecium caudatum, after Calkins and Cull, 1907. A, B, C, 

 first of the three maturation divisions. A, the numerous chromosomes 

 united in pairs lengthwise (or split?). B, the two chromosomes (or 

 halves?) separating lengthwise. C, the two chromosomes (or halves) 

 almost separated; two groups forming. D, one of the two nuclei 

 resulting from the first maturation division. E, F, second maturation 

 division. E, the two halves of the chromosomes pulling apart. F, the 

 two groups separated; the two new micronuclei united by a narrowed 

 connecting zone. G, third division (that producing the migratory and 

 stationary half nuclei). The chromosomes formed of rows of particles; 

 the rows have broken in the middle and the halves are separating. H, 

 union of migratory and stationary half nuclei, in the two individuals. 

 The oblique line is the surface of separation of the two mates. In each 

 individual the migratory half nucleus is the smaller one. 



Thus at this division each of the two half nuclei formed 

 receives a half of each of the chromosomes (then present in 

 the reduced number). The original number 1 would of course 

 be restored by the mating of the migratory -and the sta- 

 tionary nuclei (H). 



Reduction is not so well known in the other groups of 



