MONOCENTRIC MITOSIS WITH SEGREGATION OF CHROMOSOMES IN SCIARA AND ITS BEARING ON THE MECHANISM OF MITOSIS

I. THE NORMAL MONOCENTRIC MITOSIS. II. EXPERIMENTAL MODIFICATION OF THE MONOCENTRIC MITOSIS

¹ From the Department of Embryology, Carnegie Institution of Washington, and the Department of Zoölogy, Johns Hopkins University

The present paper supplements previous studies on the monocentric first spermatocyte division in Sciara and considers the findings in relation to the general problem of the mechanism of mitosis. Contrary to generally accepted principles of mitosis, an accurate segregation of chromosomes is effected in a unipolar field during the mitosis in question. The segregation is highly selective in that paternal chromosomes react in opposite fashion from their maternal homologs and pass away from the pole instead of toward it. These chromosomes which "retreat" from the poles exhibit characteristics which are believed (1) to demonstrate the functional reality of the so-called "spindle fibers," which in this case retard the movement of the chromosomes, and (2) to suggest that the opposing force or activity responsible for the movement of the chromosomes is due to an activity of the chromosomes themselves. The hypothesis of "autonomous" movement on the part of the chromosomes is suggested particularly by two lines of evidence:

(1). Each retreating chromosome in its movement directly away from the pole acts independently of the others and may move along any radial line within the theoretical cone delimited by imaginary extension of the half spindle. Its path appears to be determined entirely by its original position with respect to the pole.

(2). Each retreating chromosome appears to move in a small mass of protoplasm derived from the original half spindle, and to maintain its position at the apex of this mass during the movement. On account of the opposing activity represented by the so-called "spindle fiber," it seems improbable that the chromosome could retain this apical position if it were not itself responsible for the movement.

The suggestion is made that the anaphase movement of chromosomes may be primarily due to such activity of the chromosomes themselves and that this activity serves to bring about movement by producing localized alterations in the viscosity of the adjacent protoplasm.

II

Experimental evidence is reviewed which shows that the retreating chromosomes are alive and therefore theoretically capable of functioning as postulated during the movements in question. This evidence also bears on the interrelations between sex and chromosome behavior to be considered subsequently.