AN INTRODUCTION TO THE METAZOA 107 



Chromosome Reduction. We are able to explain now why a 

 reduction in the number of chromosomes takes place during 

 maturation. It has already been pointed out (p. 32) that 

 every species of animal has a definite, even number of chromo- 

 somes in its somatic cells. This number remains constant gener- 

 ation after generation. Now if the mature egg contained this 

 somatic number of chromosomes and the sperm brought into it a 

 like number, the animal which developed from the zygote would 

 possess in its somatic cells twice as many as its parents. The 

 number is kept constant by reduction 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 number 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 fertiliza- 

 tion may be understood. It appears that chance has very little to 

 do with the union of chromosomes in pairs during the early his- 

 tory of the germ cells (pp. 103 106, Figs. 49, 50, 51, b); 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 (p. 32) it follows that the blend- 

 ing of the characteristics of the mother and 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. 



3. EMBRYOLOGY 



Cleavage. The division of the fertilized egg is known as 

 cleavage. The chromatin of the united germ nuclei condenses 

 into chromosomes, which are so arranged on the first cleavage 

 spindle (Fig. 51,7) that each daughter nucleus receives half of 

 each. This means that each daughter cell will contain half of 

 each chromosome of paternal origin and half of each chromosome of 



