SEXUAL AND ASEXUAL REPRODUCTION 257 



the endless series of animals and plants may be found great 

 variety in the manner by which this is accomplished; but in 

 all cases some efficient device is found for bringing the egg 

 and sperm into contact. 



The egg and the sperm have a strong attraction for each 

 other, so great that when brought into each other's proximity 

 the sperm will be attracted to the egg and attach itself. 

 The head of the sperm then buries itself in the egg, as shown 

 in Figure 121 G, ra, the tail being left on the outside, but the 

 centrosome being carried in with the head. The tail has no 

 further function. This entrance of the sperm into the egg 

 may occur either before or after the changes in the egg that 

 have been described as maturation. If the sperm enters before 

 the egg is fully matured it remains in the egg in a dormant 

 condition, and is now known as the male pronucleus, until 

 after the egg has been brought into the condition above de- 

 scribed as mature, with its chromosomes reduced to half their 

 normal number." If the sperm does not enter the egg until 

 after the egg is mature, the further changes which bring about 

 fertilization occur at once. 



Fertilization. After the sperm has entered the egg and the 

 egg has become matured, the nucleus of the egg and the sperm 

 head (the two pronuclei) approach each other; Fig. 121 H. 

 What brings them together is not exactly known; apparently, 

 in some cases, the centrosome seems to have something to do 

 in bringing the two nuclei in contact, and without much doubt 

 they have an attraction for each other. At all events the egg 

 and the sperm are soon brought together and finally fused 

 with each other, forming a single fusion nucleus. This fusion 

 is the fertilization proper (sometimes called impregnation). 

 Since the egg nucleus contains two chromosomes and the 

 sperm head, or male nucleus, also contains two, when these 

 two unite the fusion nucleus evidently contains four of them, 

 and thus the number of chromosomes is restored to the same 

 number as that possessed by the ordinary cells of the body 



