MECHANISM IN SEGREGATION 49 



are now in the form of thin threads. At the point where 

 the two threads come togetlier (Fig. 20, /) they can often 

 be seen to be shaped like a Y and, at the point of meeting, 

 the uniting threads are often twisted about each other. 



The fused part of the united threads steadily grows 

 shorter and thicker. They become the condensed pachy- 

 tene threads, and appear as represented in Fig. 20, g. The 

 thick threads shorten further, and the line of fusion 

 between them (or a new line of cleavage) appears, as 

 seen in Fig. 20, li. It will be noticed also that the ragged 

 outline that the chromosomes had during the preceding 

 stages is gradually lost, so that they now appear as solid 

 rods or cords, which finally when they have reached the 

 last stage in their condensation (Fig. 20, i) appear (in 

 Batracoseps) as rods tivisted about each other. Whether 

 this twisting represents the original wrapping around 

 each other of the leptotene threads as they conjugate, or 

 whether it is a new arrangement resulting from the con- 

 densation of the chromosomes that are not free to move at 

 all points, hence twist about each other as they condense, 

 is a question that calls for further and careful considera- 

 tion. For the present — since segregation alone is here 

 involved — this matter may be laid aside. In this con- 

 densed condition the chromosomes pass into the first 

 maturation division. 



As already stated, the union of the chromosomes in 

 the eggs of the female has been less often studied, but 

 that the process is essentially the same is sufficiently evi- 

 dent. In one of the sharks, Pristiurus melanostomus, the 

 following stages described by Marechal show how similar 

 are the maturation stages in the female to those in the 

 male. When the germ-cells have reached the end of the 

 multiplication period they pass into the synaptic condition, 

 as shown in Fig. 21, a to d. Then threads appear in the 

 nucleus ; and soon it becomes evident that most of them 

 are in the form of loops, whose ends are uniting in pairs 

 (Fig. 21, e, /). When conjugation is finished thick loops 



4 



