A Quantitative Study of Chromatin Diminution in Embryonic Mitoses of CYCLOPS FURCIFER.

I T has been reported earlier (S. BEERMANN 1959) that the primordial somatic cells in some copepods become different from the germ line cells by a process of “chromatin diminution” similar to that which occurs in Ascaris (BOVERI 1899). A good example is Cyclops strenuus diuulsus. As observed by HACKER (1894) and by STICH (1954) the mitotic spindles in the 5th cleavage division of this species contain large amounts of chromatin material, and STICH (1954, 1962) has attributed this to an extra synthesis of DNA. The cytological details indicate quite clearly, however, that the “extra” chromatin simply derives from a disintegration of heterochromatic chromosome segments: In the diplotene stage of the oocytes the chromosomes of C. strenuus diuulsus all possess long terminal segments of heterochromatin. In developing eggs heterochromatin as such is not visible until the telophase of the 4th cleavage division. During prophase of the 5th division the heterochromatin is seen to become dispersed throughout the nucleus in all somatic cells. This material then fills the spindle and is finally expelled into the cytoplasm. At the same time the chromosomes undergo a drastic reduction in their length and some of them show arm ratios which are different from any of those observed prior to diminution (figures in S. BEERMANN 1959). These changes are irreversible, and no heterochromatin has been found in somatic cells following the 5th cleavage. As expected, the chromosomes in the primordial germ cell and its descendants remain unchanged, and big chromocenters are present in germ line cells at all stages of development. In C. strenuus divulsus, as in Ascaris, the observed loss of terminal heterochromatin during diminution could simply be due to a fragmentation of the chromosome regions involved. However, in Cyclops furcifer, another member of the strenuus species group, the chromosomes have heterochromatic segments not only at their ends but also in the kinetochore regions (Figure l ) , and the latter seem to undergo diminution as well (S. BEERMANN 1959; Figure 2). This would imply a more sophisticated mechanism of diminution. Furthermore, the process of diminution in this species, though similar in its essentials to the occurring in C. strenuus diuulsus, has been found to require two divisions for its completion, SO that the first and major diminution in the 6th cleavage division is followed by a second diminution in the 7th cleavage division (Figure 3 ) . These cytological findings make C . furcifer a particularly interesting species for quantitative and experimental studies on the problem of chromatin diminution. A full account of the cytological events in C . furcifer will be published elsewhere. In this paper