INHERITANCE 301 



transmission. Imagination has proceeded beyond the 

 limits of vision, and has pictured them composed of "bio- 

 phores," "ids" "determinants," and other hypothetical 

 structures, capable of handing down unit characters in 

 inheritance. The existence of these, or of any other such 

 mechanism, is not at present capable of either proof or dis- 

 proof; and need not detain us here. But we may note in 

 passing that some progress has been made in relating charac- 

 ters of the adult organism to characters of the chromosomes 

 of the germ cells. An excellent example is furnished by the 

 so-called "accessory" or sex-accompanying chromosome of 

 certain Hemiptera and other arthropods. In the squash 

 bug, for instance, in the body cells of the female there are 22 

 chromosomes; in the male, but 21. In the cells of this sex 

 one chromosome exists unpaired, all the others are joined in 

 pairs. In the maturation of the sperm mother cells, the 

 division that occurs without the previous splitting of 

 chromosomes, leaves an odd chromosome in half the cells. 

 The resultant sperm cells, odd and even in their chromosome 

 complement, unite with the full-equipped egg cells, as 

 indicated in the accompanying diagram, to produce new 

 male or female organisms, according to the chromosome 

 distribution. This accessory chromosome (fig. 179), which 

 the female zygote only receives, is sometimes, as in the plant 

 bug (Lyg&us) accompanied by a small mate, (y, of the 

 figure) , which in fertilization only the male offspring receive. 

 This is a further evidence of the connection between the 

 accessory chromosome and the sex of the adult. 



Chromosome reduction occurs, apparently, in all the 

 higher organisms, both plants and animals, but the attend- 

 ant circumstances appear not always to be the same. It is 

 everywhere a preliminary to fertilization. In- the higher 

 plants it occurs at the time of spore formation; and the 

 spores and all the cells of the gametophyte phase, as well as 



