i 9 i6] NOTHNAGEL— REDUCTION DIVISIONS 471 



both have a similar structure during the early prophase. The 

 first difference is evident when, apparently owing to some osmotic 

 force, the thread contracts into the synaptic ball. During this 

 contraction there is an increase in chromatin substance, preparatory 

 to the two rapid divisions which follow. At the time of second 

 contraction, this condition undoubtedly being due to osmotic 

 activity again, bivalents are formed. Following the third con- 

 traction, where it has been pointed out plainly that exosmosis is 

 the factor underlying spindle formation and massing of bivalents, 

 the members of each pair separate for opposite poles. Seemingly 

 this last contraction, or the 3 contractions taken together, hold in 

 check the dissociation of the halves, which in somatic mitosis 

 would have occurred at this time; for immediately following the 

 separation of whole chromosomes, the halves move apart, join end 

 to end, and, as soon as a new spindle is formed, go to opposite 

 poles. 



From this investigation it appears to the author that on account 

 of these various contractions, a regular somatic mitosis, although 

 started normally, is first varied and then checked for a time, 

 resulting in the heterotypic division, and not until homotypic 

 division is the typical mitosis completed. 



Summary 



1. During late telophase of the last division of the sporogeneous 

 tissue a row of vacuoles appears along the median longitudinal 

 axis of each chromosome, these enlarging until each member is 

 a ladder-like structure. Accompanying this there is an end to end 

 approximation. Such is the condition of the resting nucleus. 



2. The paired threads entering synapsis and there approxi- 

 mating are the two sides of the ladder, the connecting strands having 

 broken down. This process does not represent the pairing of two 

 spirems. Throughout this period the chromosomes have retained 

 their individuality. 



3. The spirem, which consists of 16 end to end chromosomes, 

 will take the form of radiating loops during second contraction, 

 segmentation occurring at the outer bend. Each of the bivalents 

 so formed consists of two somatic chromosomes that were end to 

 end in the spirem. 



