THE NUCLEUS IN MATURATION 19 



phase is especially important. During it the chromatin is gathered into a tangled 

 skein at one side of the nucleus (fig. 28, B). This may be named the synaptic 

 phase (synapsis of Moore; see p. 22). During this phase a reduction in the 

 number of chromosomes takes place by a fusion of the somatic 

 chromosomes in pairs. The manner of the fusion is disputed, some observers 

 holding that the chromosomes unite end to end ; others that they fuse along their 

 whole length. The fused rods (now one-half the number characteristic of the 

 somatic cells) go through elaborate changes, and ultimately form double rods 

 (fig. 28, a e), which take various forms (pseudotetrads, rings, &c.), according 

 to the manner in which the rods are united together. In a very small number 

 of cases (Ascaris) the prophase figures take the form of four isolated bodies or 

 tetrads. Such cases fall into a special category, and will be treated separately. 



FIG. 28. SOME STAGES IN SPEBMATOGEXESIS OF MYXINE. (Schreiner.) 



a and &, synaptic phase ; c, d, double thread stages ; e, double chromosomes (rings and double rods) ; 



/, metaphase of heterotype. 



These double-rod prophase figures (' gemini ') are gathered on to the equator of 

 the first maturation- spindle, and the resulting metaphase figures vary according 

 to the manner in which they are attached to the threads of the spindle. They 

 may be attached at their extremities, at their middle points, or nearer one end 

 (fig. 29). It follows that when the two portions of the double rods are separated 

 on the spindle, in the first case the chromosome will be a straight rod, in the second 

 a V, in the third a V with unequal limbs (fig. 29, 6). The result is the same 

 in each case : the two portions of the double rod are separated, and as these 

 portions represent not a longitudinally divided chromosome, but the two somatic 

 chromosomes which have fused in the synaptic phase, whole chromosomes are 

 separated and distributed on the spindle, instead of the halves of the longitudinally 

 split chromosome, as in ordinary mitosis. A further complication of the hetero- 

 typical division is that a cleavage of the rods manifests itself as they pass to the 

 poles of the spindle, and a double figure again results (figs. 29 c, 30 b). This cleavage 

 is preparatory to the second or homotypical division, in which the elements are 



c2 



