iqiA] NOTHNAGEL— REDUCTION DIVISIONS 457 



chromatin material has contracted considerably. As the result of 

 measuring many nuclei, only those being measured in which the 

 entire nucleus was contained in the one section, the following state- 

 ment may be made safely. Since the diameter of the contracted 

 mass is considerably less than that of the nucleus just previous 

 to contraction, or of the resting nucleus (figs. 7-12), synapsis is 

 a true contraction. The synaptic mass lies almost or entirely 

 against one side of the nuclear cavity, with the nucleolus just out- 

 side or partially held by a few strands of the spirem. 



FORMATION OF BIVALENTS 



As the synaptic mass loosens up, a comparatively thick, 

 smooth thread may be seen twisting and winding about; and 

 gradually loops free themselves, extending for various distances 

 into the cavity (figs. 13, 14). It was stated that "a comparatively 

 thick, smooth thread'' was freed from the mass; while in fact the 

 thread is irregular in outline and stains irregularly, the darker 

 places being those denser chromatin aggregations seen in the 

 ladder-like structure of the resting nucleus (figs. 3-9). 



The spirem, although double in nature, is composed of single 

 somatic chromosomes placed end to end, the double nature having 

 arisen by the vacuolization or splitting of single chromosomes 

 during late telophase of the last division of the sporogenous tissue. 



When the spirem is entirely disentangled, loops of varying 

 lengths are at first irregularly distributed within the nucleus 

 (fig. 15); later, when they are peripherally arranged, the loops are 

 longer and may be traced even entirely around the cavity, forming 

 what is known as the hollow spirem (fig. 16). The double nature 

 is rarely discernible during this latter period, since the thread 

 has condensed and grown until it appears as a homogeneous 

 structure staining uniformly throughout. Occasionally the halves 

 of the thread separate (fig. 16) and may be seen even to twist about 

 each other, the origin being in early synapsis (fig. 9, left side). 



At the close of the hollow spirem stage the thread thickens and 

 becomes entangled in the center (fig. 17), continuing until the 

 typical second contraction results; which phase consists of the 

 well known radiating loops that usually extend to the periphery 



