FACTORS IN ONTOGENY 



271 



absence of an intermediate resting stage between them, the 

 second division following immediately upon the first without 

 the reconstitution of the chromosomes into the skein stages. 

 The second peculiarity lies in the fact that the chromatin (now 

 the individual chromosomes) appears in one half the typical 



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Fig. 155. — Reduction of chromosomes in the spermatogenesis of Ascaris megalocephaJa 

 var. hivalens: A, Nucleus of a si)ermatogoniuin, the typical number of chromo- 

 somes (4) shown, each split longitudinally preceding the nucleus fission; B, young^ 

 splitting primary spermatocyte, two tetrads present, each with body double from 

 longitudinal splitting of a chromatin thread; C, the tetrads in the equatorial stage 

 of fission; D, separation of the dyads; E, the djads in succeeding fission of the 

 secondary spermatid; F, completion of the fission of the same, each cell (spermatid) 

 contains the reduced number of chromosomes (2). (After Brauer.) 



number of the chromosomes in the first division, and is 

 usually arranged in "tetrads,'^ or groups of four rounded, 

 deeply staining bodies connected by linin fibers. These tetrads 

 are always one half the number of the original rod- or thread- 

 like chromosomes. Thus in Fig. 155, A represents a sperma- 

 togonium nucleus of Ascaris with the four chromosomes, 

 showing tlic longitudinal splitting preparatory to division. 



