CROSSING OVER AND CHROMOSOMES 101 



first split, so that four parallel strands appear. It is 

 customary to call the split, that is supposed to correspond 

 to the line of union of the maternal and paternal chromo- 

 some, the primary split or reductional split, and the split 

 that corresponds to the longitudinal division within the 



Fig. 41. — Diagram showing crossing over of two chromosomes at the four-strand stage, 

 a,b, and the subsequent opening out of the tetrad, d. 



maternal or the paternal chromosomes, the secondary or 

 equational split. Only in very special cases is it possible 

 to be able to say which is the primary and which is the 

 secondary split. In fact, whenever crossing over takes 

 place in the four-strand stage this distinction fails to 



have much meaning. 



