January 8, 1909] 



SCIENCE 



57 



tion is clearly shown in the nuclei of the 

 spermatozoa, and in particular in the con- 

 stitution- of the chromosome groups. The 

 spermatozoa are in fact of two classes, equal 

 in number, that differ in respect to one or 

 more of the chromosomes that enter into 

 the formation of their nuclei ; and the facts 

 clearly demonstrate that fertilization of the 

 eggs by one class produces males, by the 

 other class females. This dimorphism of the 

 spermatozoa was discovered by Henking as 

 long ago as 1891 in the hemipteran genus 

 Pyrroclioris, and was confirmed by Paul- 

 mier in Anasa eight years later, but neither 

 of these observers suspected its meaning. 

 Its significance was first suggested by Mc- 

 Clung in 1902, but direct proof of the fact 

 was first brought forward by Stevens and 

 Wilson three years later in certain species 

 of hemiptera and coleoptera. The result 

 attained in these species has now been ex- 

 tended to nearly a hundred species of in- 

 sects through the studies of a number of 

 observers, among whom a group of Amer- 

 ican eytologists have led the way. In all 

 these species sex-production conforms to a 

 common principle, which has recently re- 

 ceived a beautiful confirmation through the 

 study of some of the parthenogenetic spe- 

 cies; but there are many variations of de- 

 tail, which have been so puzzling as to have 

 caused many errors of observation and in- 

 terpretation, and the literature of the sub- 

 ject has in consequence fallen into a bewil- 

 dering confusion that is only now fairly 

 being cleared away. I will state the essen- 

 tial facts as briefly as possible. 



In all the species half the spermatozoa 

 are characterized by the presence of a spe- 

 cial nuclear element which I shall call the 

 "X-element," while the other half fail to 

 receive this element. In the simplest and 

 clearest case (which was that first discov- 

 ered) the X-element is a single chromosome, 

 now generally known by the name of the 



"accessory chromosome," given to it by 

 McClung, but it is also called the "odd" 

 or "hetero tropic" chromosome, the "mono- 

 some," or the "unpaired idiochromosome. " 

 I will here employ MeClung's more fa- 

 miliar term. As a single accessory chromo- 

 some the. X-element has been found in 

 many representatives of the hemiptera, 

 orthoptera and coleoptera, and in certain 

 odonata, myriapoda and arachnida. The 

 typical accessory chromosome has no syn- 

 aptic mate or partner; and here lies the 

 explanation of the fact that in the matura- 

 tion divisions it passes into only half the 

 spermatozoa.^ In many cases, however, the; 

 X-element (otherwise identical with an ac- 

 cessory chromosome) appears as a "large- 

 idiochromosome" which has a synaptic- 

 mate known as the "small idiochromo- 

 some." This latter chromosome, or its: 

 homologue, I shall designate as the "Y- 

 element." In a few cases the X-elemenfc 

 consists of two chromosomes (ThyantcHf 

 Fitchia), of three (Prionidus, Sinea), or 

 even of four (Gelastocoris) , accompanied 

 in each of these cases by a single T-element. 

 In Syromastes (at present a unique case) 

 the X-element is double, but is not accom- 

 panied by a Y-element.^ In all cases the 

 spermatozoa are formed in pairs, and the 

 chromosomes are so distributed in the 

 maturation-divisions that one member of 

 each pair receives the X-element (whether 

 it consist of one, two or more chromosomes) , 

 the other member the Y-element if it be 

 present. This is illustrated by the accom- 



^ The two members of every pair of chromosomes 

 are separated in the reduction division and pass, 

 respectively, into the members of a corresponding 

 pair of spermatozoa. Hence the reduction of the 

 original number to one half in each spermatozoon, 

 and hence also the failure of one member of each 

 pair of spermatozoa to receive the X-element. 



° The cases of Fitchia, Prionidus and Sinea are 

 reported from unpublished observations by Mr. F.. 

 Payne, made in my laboratory. 



