REVIEWS. 199 



logons chromosomes come together at synapsis it has been demon- 

 strated, in some forms at least, that they twist about each other so 

 that one chromosome comes to lie now on the one side now on the 

 other of its partner. If at some points the chromosomes break and 

 the pieces on the same side unite and pass to the same pole of the 

 karyokinetic spindle, the necessary condition for crossing-over will 

 have been fulfilled." 



Prom their experiments the authors conclude that the Y chromo- 

 some (" the mate of the X chromosome in synapsis and reduction") 

 "does play some positive role," and " is necessary for the fertility of 

 the male," yet "it has no" effect upon sex itself." Equally positive is 

 the evidence that " sex is quantitatively determined by the X chromo- 

 some " — two X's determine a female and one X a male. " Sex is 

 determined by the combination of the X chromosomes, and that the 

 male and female combinations are the causes of sex differentiation, 

 and are not simply the results of maleness and femaleness already 

 determined by some other agent." 



It seems to be a fact that " mutation " is more frequent in Droso- 

 phila ampelophila than in most other subjects which have been chosen 

 for experiment. Possibly one factor in this conclusion is the immense 

 number of flies that have come under the eyes of these workers, and 

 another reason may be that the more or less artificial conditions under 

 which they are produced' may have led to the unparallelled mutation. 

 Mutations may be of two classes, " mutations through loss " and muta- 

 tions by " addition." Some quite new characters have appeared, but 

 there is, it is said, no evidence to justify them " in inferring anything 

 whatever in regard to the nature of the change that takes place in the 

 germ-plasm." 



The results of experiments with mice compelled Cuenot to the 

 suggestion that more than two factors may stand in the relation of 

 allelomorph to each other. As a confirmation of the correctness of 

 this explanation our authors state that " there are at least two such 

 systems among ths factors in the first chromosome in Drosophila.'" 

 The first of these includes the factor for white eyes, that for eosin 

 eyes, and that for cherry eyes, with the allelomorph of these three, the 

 factor for red colour present in the Avild fly — a quadruple system. The 

 second system is made up of the factor for yellow (body-colour), and 

 that for spot (on abdomen), with their normal allelomorph the factor 

 for gray in the wild fly — a triple system. It is pointed out that one of 

 the most striking facts connected with the subject of multiple allelo- 

 morphs is that the same kind of change is effected in the same organ. 



One of the most difficult questions dealt with was the consideration 

 of the character termed "lethal," under which most of the types, if 

 left to themselves with natural conditions, would soon die out. What- 

 ever may be the method of action of these " lethal factors," sufficient 

 study has not yet been given to suggest, but it can be shown " that 

 from among the offspring obtained from certain stocks expected classes 

 are missing, and the absence of these classes can be accounted for on 

 the assumption that there are present mutant factors that follow the 

 Mendelian rule of segregation, and which show normal linkage to other 

 factors, but whose only recognisable difference from the normal is the 

 death of those individuals which receive them." It was found in the 



