104 GENERAL BIOLOGY 



The diploid number of chromosomes is reduced to the haploid num- 

 ber by a union of the chromosomes, two by two. And this union in twos 

 is by no means haphazard. An understanding of this can best be seen 

 in animals where the chromosomes are different both as to shape and 

 size. The squash-bug (Anasa tritis) is a good example. 



In these bugs the chromosomes occur in two sets, larger ones and 

 smaller ones (Fig. 30). During pseudo : reduction, the larger unites with 

 a larger one, and the smaller with a smaller one, and so on. All the 

 resulting tetrads are symmetrical. 



The- sum total of all the character-factors which are received from 

 the parents of an animal at the time the egg is fertilized are contained 

 in these two sets of chromosomes. In some insects virgin birth is not 

 uncommon. In these cases a complete individual develops from the 

 mature egg alone that is, from the one having only one-half the definite 

 number of chromosomes normally present in each cell of that species. 

 This shows that each set of chromosomes contains all that is necessary 

 for a complete individual. We, therefore, think that the linking of a 

 similar chromosome from the male and a similar one from the female 

 must be for the purpose of bringing similar important factors together 

 so as to strengthen such factors. A fuller discussion of inheritance will 

 be left for the chapter on Genetics. 



FERTILIZATION 



The union or fusion of the sperm nucleus and the egg nucleus is 

 known as fertilization ( ). The spermatozoan is 



composed of three parts, head, tail, and mid-piece (Fig. 31). The head 

 is largely nuclear material and is the only portion which actually en- 

 ters the egg and fuses with it. Sperm may enter an egg either before 

 or after maturation of the egg is completed. 



After the sperm cells have passed through the maturation process 

 a great mass of them are secreted at one time from the spermaries. If 

 an animal lives in water the sperm float about in that fluid, otherwise 

 enough liquid is excreted to make it possible for the sperm to float about 

 until coming in contact with an egg. 



Among all higher animals there are special copulatory organs which 

 vary considerably in different animals but which, in all cases, serve to 

 bring egg and sperm together. 



There is a great attraction between these germ cells of the different 

 sexes which cause their union and fusion, though what this attraction 

 is has not yet been discovered. 



If the sperm enters the egg after the latter has matured (which 

 is by far the more common method) certain changes begin taking place 

 at once. 



The sperm nucleus is called tfie male pronucleus (Fig. 32) after it 

 enters the egg while the nucleus of the egg is known as the female pro- 



