26 GENETICS 



well as the increase in the size of the sperm nucleus, 

 until in Figure 19 the process is complete so that the 

 centrosomes have assumed a polar position and the 

 sperm nucleus is equal in size to the egg nucleus 

 and lies in contact with it. In Figure 20 the chro- 

 matin network of the two nuclei has formed into an 

 equal number of chromosomes which in each case 

 is half the number characteristic for the species. 

 Figure 21 shows the complete disappearance of the 

 nuclear membrane, a process that had already begun 

 in the preceding figure, and also the arrangement of 

 the chromosomes, connected with mantle fibers, in the 

 equatorial plane where the former split longitudinally. 

 In Figure 22, when the half chromosomes thus formed 

 pull apart and migrate toward the poles, the segmenta- 

 tion of the fertilized egg has begun, and there finally 

 occurs, as shown in Figure 23, the two-celled stage 

 following fertilization in which each cell contains the 

 normal number of chromosomes, half of which came 

 from the egg and half from the sperm. 



9. PARTHENOGENESIS 



Fertilization is by no means an essential process in 

 the formation of a new individual, even in those ani- 

 mals which produce both eggs and sperms. Many 

 animals and plants reproduce parthenogenetically, 

 that is, the egg-cell may develop without first uniting 

 with a sperm-cell. In these instances the chromo- 

 somes of the egg are not halved during maturation, 

 and the offspring, therefore, have the same number 



