THE GENETIC SYSTEM 39 



known as the haploid number. The number 2« (or 2«-l), 

 present in the two together, is known as the diploid num- 

 ber. In different organisms the haploid number varies from 

 I to more than lOO. 



4. In some of the fertihzed eggs (those that will develop 

 into males), in some organisms, the pronucleus from the 

 mother contains one more chromosome than that from the 



Figure 14. Fertilized egg of Ascaris megalocephala, showing the 

 two nuclei, one from the father, one from the mother. After Boveri, 

 from Wilson. Each nucleus contains two large chromosomes. 



father. This additional chromosome is known as X. In 

 the eggs that are to produce females (in these same or- 

 ganisms) the two pronuclei contain the same number of 

 chromosomes, each having an X. 



5. In organisms in which the number n is the same for 

 both pronuclei, frequently in the eggs that are to produce 

 males one chromosome (X) in the maternal pronucleus is 

 larger than the corresponding chromosome (Y) in the 

 paternal pronucleus. 



6. In some organisms the different chromosomes in each 

 single pronucleus differ much in size and shape (as in fig- 

 ures 3 and 7). 



7. But the set of chromosomes in one of the two pro- 



