254 



INTRODUCTION TO CYTOLOGY 



a set of seven visibly different chromosomes. Sakamura (1920) holds 

 the number here to be six rather than seven. (See p. 160.) 



In Vicia faba (Sharp 1914; Sakamura 1915, 1920) there are in the 

 somatic cells 12 chromosomes, two of them being about twice as long as 

 the other 10 (Figs. 56 and 102). At synapsis in the microsporocyte there 

 are formed six bivalents, one of them having about twice the length of 

 the other five. Hence it is clear that the two long chromosomes pair 

 with each other. In the heterotypic mitosis the synaptic mates separate 



SIMILAR PROCESS IN <? A X 



A -(/ 



CREPIS V1RLNS S.MH.AR PROCESS .N 



FIG. 102. Chromosome cycles in Vicia faba and Crepis virens, showing 

 homologous pairing. 



and pass to the daughter nuclei, bringing about reduction. At the close 

 of the homceotypic mitosis the microspore, and hence the male gamete 

 to which it later gives rise, receives a simplex group of six chromosomes: 

 one long and five short. Since the somatic cells contain each of these in 

 duplicate it is evident that a similar set is contributed by the female 

 gamete. 



Summing up, we may draw from the above facts certain very impor- 

 tant conclusions: (1) Each parent furnishes the offspring with a set of 

 chromosomes, the members of the two sets being intermingled in all the 

 nuclei of the new indiiidual. This point will receive further attention in 

 the following chapter on fertilization. (2) The two members of each 

 bivalent chromosome formed at synapsis are derived one from each parental 

 set. (3) Each chromosome of the paternal set conjugates with a particular 

 chromosome of the maternal set: the two are in some sense homologous. 



