574 SCIENCE PROGRESS 



the gametes ; and this reduced or haploid number is delivered 

 to the latter when ready for their union. Fertilisation restores 

 the diploid number by bringing together two haploid groups ; 

 and the diploid number thereafter persists until reduction 

 again occurs. Reduction takes place in the process of matura- 

 tion, which culminates in two maturation-divisions. In all 

 higher animals these divisions are the last two preceding the 

 formation of the gametes. In higher plants the corresponding 

 divisions occur in the sporophyte, or asexual generation and 

 give rise to asexual spores which only produce the gametes 

 after the formation of a sexual generation or gametophyte. 



(I) The sexual dimorphism of the spermatozoa in insects 

 results from an asymmetrical distribution of the chromosomes 

 that occurs in one of the maturation-divisions (spermatocyte- 

 divisions) — in some cases the first (Orthoptera, Coleoptera, 

 Diptera), in other cases the second (many Hemiptera). Even 

 closely related species may differ in the order of division ; for 

 example, in the family of Coreidae the differential division is 

 the second in most species (e.g., Anasa, Protenor), but in a 

 few cases is the first (Archimerns, P achy lis). In either case 

 the result is to produce two exactly equal classes of spermatids 

 and of spermatozoa. 



The details of the differential division vary considerably in 

 different species, but all are reducible to a common plan. The 

 most familiar case is that originally discovered by Henking in 

 the hemipteran genus Pyrrhocoris, 1 and subsequently found by 

 other observers in many other insects and in a few myriapods 

 and arachnids. In this type a single chromosome fails to divide 

 with the others, and passes undivided to one pole. Half the 

 spermatozoa thus receive one more chromosome than the other 

 half. Examples of the actual numbers are: Pyrrhocoris 12 and 

 11, Anasa, Euthoctha, Norma 11 and 10, Protenor and Alydus 

 7 and 6, Brachystola and many other Acrididae 12 and 11, Anax 

 14 and 13. Close study leaves no doubt that the chromosome 

 which thus fails to divide is always the same particular one, at 

 least in some species, where it may positively be identified by 

 its size ; in Pyrrhocoris or Protenor, for example, it is much the 

 largest of all the original chromosomes. Half the spermatozoa 

 are thus characterised by the presence of a particular chromo- 

 some that is absent in the other half; and, as will presently be 



1 Zeitschr.f. Wiss. Zool. 1891, 51. 



