412 ELMER L. SHAFFER. 



the synaptic condition of the macrochromosome pair of the 

 spermatogonial chromosomes. 



(6) Tetrads and Maturation Divisions. Stages of the early 

 prophases of the spermatocytes were quite abundant and it 

 was possible to follow the formation of the first maturation 

 tetrads. In the early prophases, the homologous threads show 

 a great variety of twisting about each other (Fig. 20). One 

 pair (A A, Fig. 20) is easily distinguishable from the others by 

 its large size and is derived from the spermatogonial macro- 

 chromosomes. Figure 18 represents the stages in the formation 

 of the definitive tetrad from this pair (Figs. 61, 62, 63, 64). In 

 the early prophases the homologous threads of the AA tetrad 

 are very long and twisted about each other. However, they 

 retain their connections at the ends, thus making the tetrad a 

 large ring if its twists were straightened out. The space en- 

 closed by this ring is the interchromosomal space which marked 

 the point of synaptic union of the threads. In the later prophase 

 stages, the large ring condenses, the threads become thicker retain- 

 ing their point of union at the ends and the interchromosomal 

 space becomes reduced in size until in the definitive maturation 

 tetrad it is reduced to a small oval slit between the two halves 

 of the ring (Figs. 18, 21, 62). In the first maturation metaphase, 

 the macrochromosome tetrad no longer appears in the form of a 

 ring, but rather in the form of a ring flattened at the poles, or 

 as two slightly bent rods whose concavities oppose each other. 

 In a similar way, the tetrad derived from the BB pair of the 

 spermatogonia goes through the formation of a ring tetrad 

 (Fig. 19), resulting in a tetrad similar to the macrochromosome 

 tetrad (A A), but approximately half its size. The other tetrads 

 show no ring formation; usually the homologous threads become 

 free at one of the synaptic ends, retaining their connection at the 

 other end, thus giving the appearance of two chromosomes 

 joined end-to-end (Fig. 20). The condensation of such tetrads 

 produces the typical dumb-bell form tetrad, with the narrow 

 portion of the dumb-bell marking the retained point of synaptic 

 union. Thus, if the point of synaptic union is retained at both 

 ends, rings like the A A and BB tetrads are produced; if the 

 synaptic union is retained at one end only, the dumb-bell type 



