Studies on Chromosomes 83 



it forms a close knot, or rounded mass, staining almost black in 

 haematoxylin, at one side of which is the idiochromosome (now a 

 condensed chromosome nucleolus). These structures lie in a 

 large clear nuclear vacuole, as shown in Photos 39 to 42. The 

 stage thus attained is the characteristic contraction phase or 

 synizesis, which in this species is extremely marked. 8 



In the safranin-lichtgriin preparation at this period the chro- 

 mosome nucleolus is, as always, intensely red. The synaptic 

 knot varies with the relative intensities of the red and green, being 

 in some preparations distinctly red, in others pure green, in still 

 others of mixed appearance. In the succeeding stage the chromatin 

 emerges from the synaptic knot in the form of separate spireme 

 threads which lose their staining capacity for haematoxylin and 

 in the double stain are again pure green (Photos 43 and 44). 

 In the middle and late growth period they are still more or less 

 green but contain red granules. In the prophases of the first divi- 

 sion they at last lose their affinity for the green and finally appear 

 pure red; but this does not occur until just before the dissolution 

 of the nuclear membrane. Since the idiochromosome always 

 retains its intense red color it may thus be followed from stage to 

 stage with great certainty. 



The study of the whole cycle of changes from the last sper- 

 matogonial division onward gives certain very definite results in 

 regard to synapsis in general, and especially in regard to the idio- 



8 Many recent writers have expressed the opinion that the synizesis stage is an artifact produced as 

 a shrinkage product, though Miss Sargant ('96) stated very explicitly that she had seen it in the living 

 cells, and this has recently been confirmed by Overton ('05). I can fully substantiate this in the case 

 of Anasa tristis. The perfectly fresh testis, gently teased apart in a Ringer's fluid in which the sperma- 

 tozoa continue their active movements, very clearly shows nearly all the features of the spermatogenesis, 

 including the number, shape and size relations of the chromosomes, their characteristic grouping and 

 behavior in the spermatocyte divisions, the double rods, crosses and other prophase figures, the spindle 

 fibers and asters, and even, I believe, the centrosomes. In this fresh material the synizesis stage 

 appears in essentially the same form as in the sections, the nuclear knot lying in a large clear vacuole. 

 These nuclei only appear in the same region of the testis as in sections, and they show a conspicuous 

 contrast to those of earlier and later stages that lie near by them. In the post synaptic stages the chromo- 

 somes, in the form of spireme threads can be seen again spreading through the nuclear cavity. These 

 observations leave no doubt in my mind that the synizesis is a normal phase of the spermatogenesis in 

 these animals, though it is not improbable that the contraction may be somewhat exaggerated by the 

 reagents. It is evident, however, from such studies as those of the Schreiners ('06) and others that 

 the synizesis does not occur in some forms. 



