94 



HEREDITY AND SEX 



(when two are present as in the female), like all other 

 chromosomes, unite in pairs at the synaptic period. 

 A recognized method of uniting is for like chromosomes 

 to come to lie side by side. 



Before they separate, as they do at one of the two 

 maturation divisions, each chromosome may be seen 

 to be split throughout the length. Thus there are 



Fig. 50. — Chromatin filaments in the amphitene stage from spermato- 

 cytes of Batracoseps. (After Janssens.) 



formed four parallel strands each equivalent to a 

 chromosome — the tetrad group. At this time Jans- 

 sens has found that cross unions between the strands 

 are sometimes present (Fig. 49). In consequence 

 a strand is made up of a part of one chromosome and 

 a part of another. Whether this cross union can be 

 referred to an earlier stage — at the time when the two 

 like chromosomes come together, when they can be 



