CHROMATOID BODY IN PENT ATOM A. 395 



are so closely associated that they might well be reckoned as 

 forming a single tetrad. In the second division X and Y are 

 uniformly coupled to form a bivalent (Figs. 14, 15); hence but 

 three chromosomes appear in polar view (Fig. 13), and each 

 spermatid receives this number. 



The relations of these chromosomes in the spermatocyte-pro- 

 phases and in the first division show some very interesting fea- 

 tures. The autosomes (bivalents) are always characterized by 

 their quadripartite or tetrad structure, being deeply constricted 

 transversely, and also longitudinally cleft, sometimes so markedly 

 that in polar view each appears distinctly double (Figs. 5, 6, 19, 

 20). This cleft, extremely conspicuous in the prophases, is often 

 less marked during the metaphase, but is again conspicuous in 

 the anaphases, when each daughter-chromosome is always dis- 

 tinctly double, as may be seen with diagrammatic clearness in 

 polar views (Figs, n, 12). The X- and F-chromosomes, on the 

 other hand, are always bipartite and only form a quadripartite 

 body when the two are in contact or closely associated. In the 

 middle prophases they lose their compact, nucleolus-like form 

 and become more or less elongated, looser in texture, and they 

 are now conspicuously split lengthwise, but show no sign of 

 transverse division. In later prophases, as all the chromosomes 

 condense, they shorten more or less but still retain, as a rule, 

 the form of longitudinally split rods at the time they enter the 

 metaphase group. They are at this time frequently in contact 

 end to end (Fig. 17), but may be quite separate, though always 

 near together. Their later history is shown in Figs. 18, af, all 

 of which are accurately represented in the same position with 

 reference to the spindle (in correspondence with Fig. 17). Of 

 this series of figures, a and b are from metaphases, the others from 

 anaphases. This series, every stage of which is shown in numer- 

 ous cases in the preparations, clearly demonstrates that the 

 original longitudinally split rods progressively shorten, and that 

 the two halves of each are then drawn apart at right angles to 

 the original long axis. Each chromosome thus assumes a dumb- 

 bell shape (a-c) and finally divides "transversely"; but it is cer- 

 tain from the earlier stages that this division is only the comple- 

 tion of an original longitudinal split. When X and Y are closely 



