38 GENETICS 



system. It includes the chromosomes; whether it includes 

 also other parts lying outside the chromosomes will be con- 

 sidered later. For the present it is the chromosomes that 

 will be followed through the life of the individuals, since 

 it is known positively that these affect the development 

 and characteristics of organisms. They will be traced, as 

 material bodies, throughout the life of the individual, and 

 into the next generation. Understanding of the action of 

 the genetic system is largely dependent on having a clear 

 picture of these matters. 



To present this history clearly and systematically it will 

 be best to set forth the main facts in a series of proposi- 

 tions, which will be further emphasized by giving to each 

 a number or letter. We begin at the earliest condition of 

 the young individual; that is, at the fertilized egg before 

 it has divided. 



A. The Genetic System in the First Stage of the 

 Individual; the Fertilized Egg 



1. As the new individual begins its separate life in the 

 form of a fertilized egg, it contains, embedded in the cyto- 

 plasm of the cell, two nuclei, each with a set of chromo- 

 somes (figure 14). 



2. One of these nuclei is from the mother (the "female 

 pronucleus"), the other from the father (the "male pro- 

 nucleus"). 



3. The two pronuclei contain the same number of chro- 

 mosomes (or that from the father contains one less; see 

 the following) . The number contained differs In different or- 

 ganisms; it will be convenient to call the number n. Thus 

 each of the two pronuclei has a set of n (or n-i) chromo- 

 somes, and the entire egg has two sets, making in (or in 

 the male, 2«-i) chromosomes. 



The number n, present in each of the two pronuclei, is 



