162 



THE THEORY OF THE GENE 



cal, or so nearly alike that they conjugate. It is not ob- 

 vious why the other sets do not conjugate, unless, as 

 Tackholm suggests, each set of 7 has come from a differ- 

 ent wild species by crossing. The additional chromosomes 

 arising in this way are sufficiently different from the 

 original set and from each other to interfere with conju- 

 gation. 



Fig. 94. 



Maturation division of egg-cell of rose. All the single chromo- 

 somes move to one pole where they are joined by half of the con- 

 jugants. (After Tackholm.) 



Two other hybrid forms may be mentioned; in both 

 there are 14 bivalents and 7 single chromosomes. In these 

 there are twice as many conjugating chromosomes as in 

 the former hybrids. 



In only a few hybrids of the canina group is the history 

 of the chromosomes in the embryo mother sac (where the 

 egg develops) described (Fig. 94). There are 7 bivalents 



