THE CARRIERS OF THE HERITAGE 227 



in each case is half the number characteristic for the 

 species. Figure 65 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 66, 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 67, 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. 



8. 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 sperm. Many 

 animals and plants reproduce parthenogenetically, 

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

 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 of 

 chromosomes as the parent, since they are simply 

 fragments of the parent. 



Professor Loeb, by the use of certain chemicals, 

 has succeeded in doing artificially what apparently 

 is not ordinarily accomplished in nature, namely, mak- 

 ing an egg that normally requires fertilization develop 

 parthenogenetically. 



