BIGELOW: NUCLEAR CYCLE OF GOXIOXEMUS MUKBACIIII. 319 



length. In division the thickened ends of the dumb-bells gradually 

 draw apart (Figs. 62-64) ; they finally separate in the middle, although 

 the tapering ends of the two daughter chromosomes often persist for a 

 lmig time, as in the cell represented in Figure 64. As a rule, one or 

 two of the chromosomes precede the others (Figs. 62, 63, 64). In some 

 cases, however, division is more rapid and nearly simultaneous, the 

 daughter chromosomes, here oval and sharply outlined, forming typical 

 daughter plates. In the few instances in which it has been possible to 

 count them there were apparently twelve in each daughter plate (Fig. 

 67). In any event, the chr'omosomes are distinctly larger than those of the 

 spermatogonia! divisions. Although this is the ordinary type, observed 

 in about 175 cases, I have in perhaps fifteen or twenty instances noted 

 spindles presenting the appearance shown in Figures 65 and 66. The 

 splitting of the chromosomes, instead of being a gradual process, is here 

 evidently rapid, so that there is no trace of the formation of " dumb- 

 bells." The chromosomes are, on the other hand, rather irregularly ar- 

 ranged in the equatorial region of the cell, seldom or never forming a 

 typical plate ; accurate counting has been very difficult, but in every case 

 there were considerably more than twelve. The migration of the chro- 

 mosomes toward the poles is not simultaneous. It is very certain that 

 the chromosomes in such a stage as is shown in Figure 66 have already 

 undergone division : they show no trace of a double nature, and as in 

 the type described above are larger than the spermatogonial chromo- 

 somes. Finally, the diameter of the spindle is considerably greater than 

 in the more usual form, the maximum observed being 5 ft ; this, how- 

 ever, is of itself not a very important feature, since an unbroken grada- 

 tion occurs in this respect. 



As to the relation of this type of mitosis to the usual process of 

 the first maturation division, the following evidence is of importance. 

 In the first place, its comparative rarity (among several thousands of cells 

 I have observed it at most not above twenty times) argues against its 

 representing a determinate phase of spermatogenesis. There is also in- 

 ternal evidence for regarding it as merely a variation from the ordinary 

 process. I have been able to find but a single line of development in 

 the prophase, and the functional spermatids and spermatozoa are of but 

 a single sort. The chromosomes in this, as in the more usual condition, 

 are distinctly larger than in the spermatogonia. We cannot, I think, re- 

 gard these spermatocytes as the parents of the giant spermatids, to be 

 described later, for if they were, we should expect to find traces of a 

 multiple nature, either in their size or in the number of centrosomes ; but 



