STUDIES ON CHROMOSOMES. 381 



forming a spheroidal and rather dense mass separated from the 

 nuclear wall by a considerable clear space (cf. Gross's figures of 

 this stage in Pyrrhocoris, '07). 



5. Stages h to j. The prophases 



a. The bivalents. At the end of the diffuse period the nuclei 

 undergo a rapid change which marks the appearance of the defini- 

 tive prophase-chromosomes. This is accompanied by a progres- 

 sive condensation and increase of staining capacity, which reaches 

 a climax in the final prophases, and by the disappearance of the 

 plasmasome. These changes may be studied to better advantage 

 in smears than in sections, and is better shown in my material 

 of Protenor than in the other forms. As seen in sections, the 

 initial stage (figs. 105, 106) shows the nuclear threads more dis- 

 tinct, less crowded and straighter, often giving an appearance 

 somewhat similar to the beginning of Stage g, but the bivalents 

 are not yet defined. In smears of Protenor (figs. 115 to 117) it 

 is clearly apparent that the threads are separate, single (i.e., not 

 longitudinally split) and much contorted. A little later the threads 

 are seen to be forming themselves into the characteristic bivalent 

 figures, still in a very diffuse and irregular form, but plainly show- 

 ing their individual boundaries, and in some cases also their char- 

 acteristic forms (figs. 107, 108, 118, 119, photo. 17). In Pro- 

 tenor the m-chromosomes are first clearly seen at this time but 

 are much less definite in contour than in the following stage. As 

 the condensation proceeds the bivalents become more definite 

 in shape and can be more readily analyzed. In Stage i (figs. 

 109-14, photos. 18 to 23) they have the forms which have been 

 familiar to us since the early work of Paulmier ('98, '99) on the 

 Hemiptera. The most characteristic of these is (1) the double 

 cross, consisting of four arms, at right angles to each other, and 

 longitudinally split. The four arms may be equal in length. 

 More commonly one pair is shorter than the other. In the later 

 stages the four arms typically lie in the same plane. In earlier 

 ones they are often curved ; and the two longer arms may be curved 

 towards each other until they nearly meet to form a ring. (2) 



