AN INTRODUCTION TO THE METAZOA 85 



completes the process usually known as fertilization. In this 

 process the chief aim so far seems to be the union of two nuclei, 

 one of maternal origin, the other of paternal origin. We shall see 

 later that fertilization is really not consummated until the ani- 

 mal which develops from the egg has become sexually mature. 



Chromosome Reduction. — It is now possible to point out 

 the result of the reduction in the number of chromosomes which 

 takes place during maturation. It has already been stated 

 (p. 16) that every species of animal has a definite, even number 

 of chromosomes in its somatic cells. This number remains con- 

 stant, generation after generation. Now if the mature egg con- 

 tained this somatic number of chromosomes and the sperm 

 brought into it a like number, the animal which developed 

 from the fertilized egg would possess in its somatic cells twice 

 as many as its parents. The number is kept constant by re- 

 duction during the maturation divisions, so that both egg and 

 sperm contain only one half the number in the somatic cells. 

 The union of egg and sperm again establishes the normal num- 

 ber of chromosomes possessed by the parents. 



Union of Chromosomes in Fertilization. — If we return 

 for a moment to the subject of maturation, the final process in 

 fertilization may be understood. It appears that chance has 

 very little to do with the union of chromosomes in pairs during 

 the early history of the germ-cells (pp. 81-84, Figs. 47, 48, 49); 

 but that one chromosome of each pair came originally from the 

 egg and is therefore maternal, while the other was derived from 

 the sperm, and is paternal. Since the chromosomes are recognized 

 as the bearers of hereditary qualities, it follows that the blending 

 of the characteristics of the mother and the father in the germ- 

 cells does not occur when the sperm enters the egg, but when 

 the individual developing from the zygote becomes sexually 

 mature. 



(4) Embryology. — Cleavage. — The division of the fer- 

 tilized egg is known as cleavage. The chromatin of the united 

 germ nuclei condenses into chromosomes, which are so arranged 



