144 



M. W. 1U.ACKMAN. 



The chromosomes of the third type arise from thread-like 

 structures similar to those from which type A and B arise. This 

 thread may be either approximately straight, or it may be curved 

 slightly in various ways, but is never bent at a sharp angle at the 

 point of synapsis. The filament undergoes a longitudinal cleav- 

 age just as with the other types. The two resulting threads, as a 

 usual thing, lie parallel to each other (Fig. Ill, ft, c) but in some 



a 



8 



IV. 



XI 



a 



FIG. III. Evolution of the double-rod-shaped tetrads, a, bivalent chromatin 

 segment; b, c, d, formation of tetrad; e, f, tetrads as seen in early stages of the 

 spindle; g, i, ordinary tetrad in two stages of longitudinal division; h, rod-shaped 

 tetrad apparently in act of transverse division; j. dyad as seen in metaphase 

 (secondary spermatocyte). 



FIG. IV. Variation of double-rod-shaped tetrad. In early prophase the double 

 chromatin segment is often twisted as shown in b. The shortening of chromatin 

 thread results in less and less twisting; so that the two parts of the metaphase 

 chromosome merely overlie each other at an angle or are only partially wrapped 

 about each other. 



cases they are .twisted about each other, so as to form a rope-like 

 structure (Fig. IV, ft, r). In such cases the resulting chromosome 

 has a somewhat different shape. In this type of chromosome the 

 tetrad resulting appears rod-shaped or double rod-shaped de- 

 pending upon the angle from which the structure is viewed. 



