EFFECTS OF AMPHIMIXIS ON THE (JERM-PLASM 24 1 



cannot be composed of a row of loosely-connected spherical 

 bodies, each containing only a portion of its determinants. 

 Moreover, the fact that the number of idants is on the whole 

 a small one, speaks against their being regarded as ids : the 

 phenomena of reversion alone, it seems to me, require the 

 assumption of a larger number of ids. 



The chromosomes are not, it is true, in all cases rod-like, and 

 may have a more spheroidal form ; the existence of microsomes 

 has, moreover, not been definitely proved in all cases. We might 

 therefore be inclined to look upon the chromosomes as structures 

 which are not always and absolutely equivalent, and to regard 

 some of them as single ids, and others as rows of ids. This 

 conception receives support from the fact that a considerable 

 variation as regards the number of chromosomes is seen in 

 nearly allied species, in which we might expect the processes of 

 heredity to occur in almost the same way. Thus, for instance, 

 the usual number of nuclear rods in Ascaris Imiibricoides is 

 twelve, and in Ascaris inegalocepJiala two or four ; in other 

 worms belonging to the same order the normal number of rods 

 may be eight, twelve, or sixteen. I should not, however, con- 

 sider these differences sufficiently great to warrant the assump- 

 tion that these rods have a different value in different cases ; 

 and this view receives support from the observations of Boveri 

 and Oscar Hertwig, which prove that in the same species 

 (^Ascaris jjiegalocepJiala) two varieties occur, in one of which two, 

 and in the other four, nuclear rods are present in the cells. In 

 this case, then, the one variety likewise possesses twice as many 

 microsomes as the other ; and although it is not always easy to 

 determine the number of microsomes in the case of other 

 Nematodes, we may infer their existence from the form of the 

 idants. For these reasons I am inclined to regard the inicro- 

 sojiies as corresponding individually to ids, a)id the 7inclear rods 

 as representing groups of ids ', for this reason I have called them 

 idajds. 



The number of idants, and even that of the ids contained in 

 each of them, is a definite one for each individual species, but it 

 varies considerably in different species. Each id of any particular 

 germ-plasm could direct the entire ontogeny if it were present 

 in sufficient numbers ; that is to say, every id contains all the 

 determinants required for one individual : but, as has already 

 been remarked, the ids contained in the idants of a species 



