THE MECHANISM OF ASSORTMENT 75 



whether random assortment or correlated movement takes 

 place. Observation shows that sometimes one, sometimes 

 the other, member of the pair goes to the same pole as the 

 sex-chromosomes. 



It happens that in a species studied by Miss Carothers 

 {T rimer otro pis suffiisa) there are several chromosomes 

 that may show constantly terminal or subterminal attach- 

 ment of the fibres; as many as seven out of the twelve 

 chromosomes of the first spermatocyte division may con- 

 sistently show this difference. In other words, any one of 

 these seven chromosomes may have one or the other kind 

 of attachment. Each grasshopper may have any one 

 of ten of its pairs showing combinations of these kinds of 

 attachment, but of course in any one individual only two 

 possible arrangements exist for a given pair of chromo- 

 somes. It is to be remembered that for a given combina- 

 tion all the cells of an individual are exactly alike, which 

 incidentally is a strong argument in favor of the individ- 

 uality of the chromosomes. An example will give further 

 details. In Fig. 32 are shown eight groups of chromo- 

 somes {h, c, d, e, f, g, h, j) from the same individual. Each 

 group of 12 chromosomes comes from a single cell about 

 to divide. Each series of 12 is here arranged in a single 

 horizontal line. The dividing chromosome is a tetrad, one 

 of whose halves is about to separate. It is significant to 

 note that in this case the separating halves represent the 

 two conjugating members of each pair; in other words, 

 the reduction division is taking place. In this individual, 

 four of the tetrads (9-12) has subterminal attachment 

 only, i.e., for both members of the dividing pair (dyad) ; 

 four of the tetrads (Nos. 2, 3, 5 and 6) have terminal 

 attachments only, while the remaining three tetrads (Nos. 

 1, 7 and 8) have one end with a terminal attachment, and 

 the other subterminal. In addition there is the sex- 

 chromosome (No. 4) that is here going upwards toward 

 the top of the figure, and will pass with the upper half 

 of each tetrad into an imaginary cell above (the female- 



