264 



INTRODUCTION TO CYTOLOGY 



In the prophase of the second meiotic mitosis the condensing chromo- 

 somes appear characteristically in the form of threads or rods, still 

 associated in dyads at their attachment regions but diverging widely 

 elsewhere (Fig. 155). They thus present a marked contrast to the 

 closely parallel halves of split chromosomes seen in a somatic prophase. 

 By the end of prophase II their matrices have become conspicuous about 

 their chromonemata, and as the spindle develops they take their places 

 with their attachment regions in its equator. Often they appear con- 



FiG. 155. — Stages in meiosis in Gasteria. a, diplonema. b, dyad in telophase /. c, 

 interkinesis. d, chromonema in early prophase //, showing doubleness (one chromatid), 

 e, /, prophase 77; note association of chromatids by spindle-attachment regions, g, 

 metaphase 77; chromonema doubled in each chromatid, h, late anaphase 77. i, telophase 

 77. (After W. R. Taylor, 1931.) 



siderably longer and more slender than in the first mitosis and hence tend 

 to resemble somatic chromosomes in this respect. In the anaphase 

 the two chromatids constituting each dyad move toward opposite 

 poles, their shapes at this time depending chiefly upon the position of 

 their attachment regions. After reaching the poles at the end of the 

 anaphase each of the four groups of chromatids begins the telophasic 

 organization of a nucleus which passes into the metabolic condition. 



Since each chromatid is now a complete and independent chromosome, 

 it should be evident that each of the four nuclei contains a single mono- 

 ploid set of chromosomes, the set being composed of one chromatid from 



