20 PROBLEMS OF EVOLUTION 



observation shows that they are the essential part, the asti 

 sphere merely supplying motive power. 



" The illustrations on page 14 will make what follows eas: 

 to understand. 



" When the egg is maturing we see that the nucleus 

 divided by an elaborate process, and one resultant half, t 

 first polar body, is expelled from the cell. After this the 

 follows a repetition of the process, and a second polar bo 

 is thrust out. The former of these divisions has for its obj( 

 the rearrangement of the particles that make up the germ-plas 

 that part of the nucleus by which heredity is maintained, a 

 which is formed of the various chromatin loops. 1 The seco 

 division reduces the amount of plasm and reduces the number 

 hereditary characters to one-half. This reduction must ta 

 place, since the egg is to be fertilised without any increase 

 the number of the essential particles. When parthenogene 

 (birth from unfertilised eggs) takes place, for instance, 

 aphides and bees, only one polar body is got rid of: t 

 division of the nucleus that results in this is a rearran^ 

 ment of material with a view to variation, since without var 

 tion there would be no evolution. No second polar body 

 extruded, since whenever parthenogenesis is to take ph 

 there is no need to reduce the plasm to half its amount, 

 has been held that the ejection of the second polar bo 

 leaves the egg female in character, whereas formerly 

 had contained the elements of both sexes. The fact tl 

 male traits are often handed down through the fema 

 and vice versa, disproves this idea. It is true that the e: 

 has sexual characteristics (such as food for the embry< 

 but the germ-plasm has none. Sex is a secondary characi 

 in the offspring. Experiments on tadpoles, caterpillars a 

 rotifers have shown that food and temperature are factors 

 determining it. 2 



" If the egg is the scene of two nuclear divisions, one leadi 



1 On this see Maupas : Archives de Zoologie exper. et generate, vol. vii. p. : 

 (.889). 



2 In the case of tadpoles further evidence is needed to substantiate this. 



