84 THE GERM-PLASM THEORY 



are wanting, and they are so because a longitudinal 

 splitting of this cylindrical and relatively thin 

 animal never occurs under natural conditions, 

 and thus could not be provided against by nature." 



Here again we seem to be within measurable dis- 

 tance of our old friend the virtus dormitiva. 



Mention has just been made of the word amphi- 

 mixis, a state of affairs in which Weismann finds 

 another cause of variation. It is well known that 

 whilst most forms above the rank of unicellular 

 organisms multiply bi-sexually, parthenogenesis 

 occurs in other forms, and, it may be added, in a 

 rather wide range of forms. 



In the ova of both forms certain changes take 

 place before development or even fertilization 

 in the cases where bi-sexual development or amphi- 

 mixis is the rule can commence. These changes 

 consist in the extrusion of part of the nuclear 

 material in the form of what are known as " polar 

 bodies." Two of these are cast out in cases of bi- 

 sexual, one in parthenogenetic development. Now, 

 according to Weismann, this process relieves the 

 ovum of a certain number of particles of ancestral 

 germ-plasm, chance determining which may be 

 taken and which left. Thus in one descendant the 

 germ-plasmata of a, e, i, o, u ancestors may be got 

 rid of, in another those of v, w, x, y, z. The result 

 will be that the two descendants will vary in ac- 

 cordance with the kind of germ-plasms which 

 they retain or lose. Hence amphimixis, or bi- 

 sexual development, is, according to his reading 

 of the facts, a mechanism for causing variation. 



