The Cellular Basis 177 



The fact that inheritance units from the two parents unite in 

 fertilization and, later segregate in the formation of gametes, so 

 that the latter are pure with respect to any character, is a familiar 

 part of Mendelian inheritance (Fig. 59). Even if these units be 

 regarded as physiological processes they must be associated with 

 particular structures, since function and structure are inseparable 

 in life processes. What are the units in terms of cell structures 

 and where are they located in the cell? 



i. Chromosomal Inheritance Theory. We have in the chro- 

 mosomes, as Wilson especially has emphasized, an apparatus 

 which fulfils all the requirements of carriers of Mendelian fac- 

 tors (Fig. 60). (Both factors and chromosomes come 'in equal 

 numbers from both parents; both maternal and paternal factors 

 and chromosomes pair in the zygote and separate in the gamete 

 as shown in Figs. 59 and 60; and so far as is known the chro- 

 mosomes are the only portions of the germ cells which ful- 

 fil these conditions. The association segregation and distribu- 

 tion of Mendelian factors and of maternal and paternal chromo- 

 somes are exactly parallel and it is not reasonable to suppose that 

 this remarkable coincidence is without significance. 



Furthermore there is much additional evidence that the chro- 

 mosomes are important factors in heredity and development : 

 Boveri has studied the abnormal distribution of chromosomes to 

 different cleavage cells in doubly fertilized sea urchin eggs and has 

 found evidence that the hereditary value of different chromosomes 

 is different. McClung, Stevens, Wilson and others have dis- 

 covered that the determination of sex is associated with the pres- 

 ence or absence of a particular chromosome, the X or Y chromo- 

 some, in the spermatozoon which fertilizes the egg. If an egg 

 is fertilized by a sperm which lacks the X chromosome a male is 

 produced, if fertilized by the other type a female results. This 

 correlation between the presence or absence of a whole chromo- 

 some and of a developed character such as sex, is the only case 

 of the kind that is known and more than anything else it has 

 served to prove that the chromosomes contain the Mendelian 



