42 THE MAIN CURRENTS OF ZOOLOGY 



stain deeply with micro-chemical dyes. These are 

 believed to be the bearers of heredity. 



Since the germinal cells of plants and animals 

 must be fertilized before they develop, we find in the 

 chromosomes the source of maternal and of paternal 

 qualities because fertilization is essentially a union 

 of the chromosomes of egg and sperm. Eggs in 

 getting mature lose one-half their chromosomes, 

 sperms, or their fertilizing agents, also in course of 

 formation have their chromosomes reduced by one- 

 half. Now the egg is fertilized by the union of the 

 sperm with it, and the fertilized egg is bi-parental. 

 One-half the chromosomes of the fertilized egg come 

 from the mother and one-half from the father. The 

 maternal chromosomes (of the egg) and the paternal 

 (of the sperm) are the bearers of the hereditary 

 qualities of both parents. In considering the cell- 

 theory it was pointed out that the problems of he- 

 redity are at bottom cellular problems and the state- 

 ments just made will help to illustrate that point. 



In closing this review we may re-affirm, that, on 

 account of their wide influence on the entire field of 

 biology, the five events designated all qualify as 

 outstanding biological advances of the nineteenth 

 century. 



