The Cellular Basis 



183 



causing two spermatozoa instead of one to enter an egg. In 

 such doubly fertilized eggs Boveri discovered that different cleav- 

 age cells receive a different number of chromosomes and in gen- 

 eral those cells which receive the largest number develop most 

 typically, while those which receive a small number develop atypi- 

 cally. By a skillful analysis Boveri proved that normal develop- 



PRIMARY BOM -DISJUNCT ION IN MALE 



PRIMARY HOB-DISJUNCTION JH FEMALE 



.SYGOTES 



FIG. 60 a. DIAGRAM OF NON-DISJUNCTION OF THE SEX CHROMOSOMES in 

 the Maturation of the Egg and Sperm of Drosophila melanogaster, with 

 the resulting types of Zygotes (According to Bridges). Primary non- 

 disjunction in the sperm leads to the production of XXY females and 

 XO males; primary non-disjunction in the egg leads to four combina- 

 tions, two of which (XXX and YO) are non-viable; secondary non- 

 disjunction occurs in the maturation of an XXY egg. In Drosophila Y is 

 an "empty chromosome," i.e., it appears to contain no genes, yet it has some 

 influence since a male without it (XO) is sterile. Zygotes are matro- 

 clinous or patroclinous depending upon whether they get a larger number 

 of X chromosomes from the egg or from the sperm. 



