198 THE entomolog-ist's record. 



authors first point out that the significant discovery of Mendel was not 

 the 8 to 1 ratio, bub the segregation of the characters (or rather of the 

 germinal representatives of the characters). When the characters 

 which form the allelomorph pair meet in the hybrid and the germ- 

 cells are formed " the factors set/refiate from each other ivithout having 

 been contaminated one by the other.'' It is also pointed out as a corollary 

 of this discovery that when two or more pairs of factors meet in the 

 germ-cells there is an assortment with definite F„ ratio results, such as 

 9:3:3:1 (for two pairs), 27 : 9 : 9 : 9 : 3 : 3 :"3 : 1 (for three pairs), 

 etc. But although these particular, ratios do not hold good when 

 linkage takes place. Professor Morgan emphasises the fact that segre- 

 gation still holds for each allelomorph. Other ratios result in the Fg 

 generation when certain characters enter a cross together from the 

 same parent, and their factors tend to pass into the same gamete of 

 the hybrid. 



From the beginning of the microscopic study of the maturation of 

 the germ-cells, it was tempting to interpret the processes witnessed as 

 the visible means by which factors are segregated. If the number of 

 genetic factors in an animal was never greater than the gametic num- 

 ber of chromosomes we should conclude at once that each chromosome 

 carried one genetic factor. But in fact the genetic factors in most 

 cases greatly exceed the chromosomes in number, yet there must be 

 some correspondence between them, for Professor Morgan tells us that 

 the characters of the Drosophiln that have so far been investigated, 

 more than a hundred in number, fall into four groups, " the members 

 of each group being linked, in the sense that they tend to be trans- 

 mitted to the gametes in the same combinations in which they entered 

 from the parents." " A most significant fact in regard to the linkage 

 shown by the Drosophila mutants is tha,t the number of livked (f ran ps 

 corresponds to the number of pairs of the chromosomex," (four in 

 Drosojihila). 



In the discussion of the chromosomes and their influence in sex 

 production, it should be remembered that many insects have an even 

 number of chromosomes in the female and an odd number in the male. 

 Say the female has 2n chromosomes, then the male has 2« — 1. At 

 "reduction," during germ-cell formation, when eggs and sperms are 

 made, the eggs get w chromosomes each, but the sperms get either n 

 or w — 1. Those which get n have thefore a chromosome over and 

 above those which the others get, and this is the X chromosome. 

 Eggs fertilised by this latter kind of sperm therefore have 2 X chro- 

 mosomes (for every egg before fertilisation has one X chromosome), 

 and they become female. Eggs fertilised by the other kind of sperm 

 without the X chromosome (having n— 1), contain only one X and 

 become males. The authors state that they consider that " there is 

 direct experimental evidence of such a nature that there can no longer 

 be any doubt that the X chromosomes are the carriers of certain gens 

 that we speak of as sex-linked." 



The authors go into the question of " crossing-over " of the chro- 

 mosomes at the time of the germ-cell formation, when equivalent 

 exchange occurs between pieces of the chromosomes, and although 

 " the genetic evidence forces one to accept crossing-over between the 

 sex chromosomes in the female, that evidence gives no clue as to how 

 such a process is brought about." But they add, " When the homo- 



