340 



INTRODUCTION TO CYTOLOGY 



may now turn to the cytological basis of the Mendelian phenomena, and 

 inquire what visible mechanism there is in the cell which will in any way 

 help us toward an understanding of the striking behavior of the Mendelian 

 characters. 



The behavior of the chromosomes at the critical stages of the life 

 cycle as described in the chapters on reduction and fertilization must 

 first be recalled. (See Fig. 131.) It has been shown that their history 

 is as follows. Each parent furnishes the offspring with a set of chromo- 

 somes, the two sets (represented in the diagram by A BCD and abed) 

 being associated in all the cells of the offspring. When gametes (or 

 spores followed later by gametes in the case of higher plants) are to be 



FERTILIZATION 

 Onion of simplex groups 



CLEAVAGE 

 Duplex groups 



ABGD abed 



SOMATIC DIVISIOHS 

 Duplex groups 



GERM CELLS 

 Simplex groups 



FIG. 131. Diagram showing tho history of the chromosomes in the typical life cycle of 

 animals. (After Wilson, 1913.) See also Fig. 77. 



formed by the new individual the chromosomes pair two by two (synap- 

 sis), the two homologous members of each pair coming from the two 

 parental sets. In the first maturation division (usually) the two members 

 of each pair separate and enter different daughter cells: this is reduction, 

 or the separation of entire chromosomes, presumably qualitatively differ- 

 ent, instead of qualitatively similar halves of chromosomes as in somatic 

 division. In the second maturation division all the chromosomes split 

 longitudinally (equationally), so that as the result of the two divisions 

 there are four gametes (or spores), two of 'them differing from the other 

 two in chromatin content. The somatic chromosomes are therefore 

 segregated into two unlike groups: each gamete (or spore) has a single set 

 of chromosomes, the set being composed of one member of each of the 

 pairs formed at synapsis. This set represents the contribution made to 

 the following generation. 



