36 



HEREDITY AND SEX 



are brought into relation with a spindle-shaped system 

 of Unes, and move apart along these lines to opposite 

 sides of the cell. The protoplasm of the cell next con- 

 stricts to produce two daughter cells, each containing 

 a group of daughter chromosomes. 



E-f 







• 





Fig. 19. — Fertilization and polar-body formation of Nereis. The 

 four smaller figures show entrance of sperm. The extrusion of the first 

 polar body is shown in lower left-hand figure and of the second polar body 

 in the two large right-hand figures. The last three also show the formation 

 of the sperm asters, which is the beginning of the first cleavage spindle in 

 the egg. (After F. R. Lillie.) 



The egg is also a cell, and in its earlier stages contains 

 the same number of chromosomes as do the other cells of 

 the body; but after two peculiar divisions that take 

 place at maturation the number of the chromosomes is 

 reduced to half. 



