114 METAZOA IN GENERAL 



from the cytoplasm of the egg cell. Then the two pronuclei approach 

 each other. At the same time the centrioles which were brought in 

 with the sperm cell become active and a spindle is produced near the 

 center of the egg cell. The two pronuclei meet at the equator of this 

 spindle. Chromosomes are formed in each, the nuclear membranes 

 disappear as in an ordinary mitosis, and the two sets of chromosomes 

 gather on the equator of the spindle, producing an amphiaster stage. 

 Then the steps which are seen in ordinary mitosis occur in regular order, 

 including metaphase, anaphase, and telophase, the final result being a 

 division of the cell. This division initiates the development of the 

 embryo. Fertilization may be said to be completed when the sperm 

 cell enters the egg cell, when the two nuclei fuse, or, in the case last 

 described, when the two nuclei cease to retain their identity and the 

 chromosomes which develop from them come to lie in the equatorial 

 plane of the spindle. 



In either of the cases described above, the chromosomes from the 

 two parent cells appear clear and distinct and when they divide in 

 the metaphase, each of the two groups of chromosomes which pass to the 

 two poles of the spindle is half maternal and half paternal in origin. 

 When at the end of the telophase the nuclei of the two daughter cells 

 enter into a resting condition, these chromosomes lose their identity; 

 but in each cell division which will follow in the development of the 

 individual which is to be produced, the maternal and paternal chro- 

 mosomes again appear. Thus the individual represents a minghng of 

 the characteristics of the two parents, and each cell in the body has 

 this mixed inheritance. As might be expected, the steps given above 

 are varied in many ways in different types of animals but the essential 

 facts remain the same. 



140. Chromosome Reduction. — It now becomes evident why chromo- 

 some reduction occurred in gametogenesis. Every species of animal has 

 a characteristic number of chromosomes, a number which is found in 

 every somatic cell in the body and remains constant generation after 

 generation. This number seems to have no relationship to the structure 

 of the animal or to its rank in the scale of animal life. For instance, 

 there are 2 chromosomes in a parasitic worm (Ascaris) found in the horse, 

 8 in the fruit fly, 28 in the spotted salamander, 48 in man, and 208 in 

 two species of crayfish. If the numbers of chromosomes were not 

 reduced in the maturation of sex cells, the fertilized egg cells would 

 contain twice the number possessed by the cells of the parents, and 

 their number would continue to double with each succeeding generation. 

 Chromosome reduction, however, results in passing on the same number 

 from one generation to the next. In all references to chromosomes up to 

 this point only those which act as mates in synapsis have been con- 

 sidered. There are odd chromosomes which in meiosis pass to either 



