194 COMPARATIVE PHYSIOLOGY 



not only by a definite number but a definite configuration. 

 It is possible to distinguish among the chromosomes pairs 

 of different sizes and shapes (this is true of man, and many 

 mammals) ; and the maintenance of this constant configuration 

 implies that when reduction takes place one member of each 

 pair passes into each gamete. In other words, the chromosomes 

 are present in the fertilised egg in pairs, segregating in the 

 formation of the gametes into maternal and paternal com- 

 ponents, one member of each pair and one only being repre- 

 sented in each gamete. 



2. The material units on v/hich hereditary transmission 

 depends are associated in groups, the members of which are 

 independently segregated with reference to members of other 

 groups. There are four such groups in Drosophila ; and in 

 Drosophila there are exactly four pairs of chromosomes. No 

 organism is known in which the number of linkage groups 

 is numerically greater than the number of pairs of chromosomes. 



3. Lastly, we have seen that with respect to one group of 

 linked characters the sexes are differently constituted. Sex- 

 linked inheritance has been described in several groups of 

 the animal kingdom, including mammals ; there are several 

 well-established cases in man, v/here, as in Drosophila and the 

 cat, it is the male that produces two types of gam.etes. In 

 several hundreds of animal species from the most widely 

 divergent groups it is now established that one pair of chromo- 

 somes which is equally paired in one sex is represented in 

 the other sex by a single member, or a pair of unequal elem.ents. 

 For instance, in the cockroach the male has thirty-three 

 chromosomes (Morse) and the female has thirty-four chromo- 

 somes (Hogben). The female produces eggs which contain 

 seventeen chromosomes, while the male produces sperms 

 half of which possess sixteen and half seventeen chromosomes. 

 Clearly, if a sperm of the former type fertilises an egg the 

 resulting zygote v/ill have the chromosome number (33) 

 characteristic of the male, while if the latter type fertilise 

 an egg the resulting zj^gote will have the female constitution 

 (34). Drosophila, on the other hand, is a case where one pair 

 of chromosomes (XX) similar in the female is represented by 



