S6 CEI.l -DIVISION 



appears truncated at the ends, its fibres terminating in a transverse 

 row of granules (maturation-si)indles of Ascaris, and some plant-cells). 

 It is not entirely certain, however, that such spindles observed in 

 preparations represent the normal structure during life. 



b. The Chromatic Figure. — The variations of the chromatic 

 figure must for the most part be considered in the more special 

 parts of this work. There seems to be no doubt that a single 

 continuous spireme-thread may be formed {</. p. 113), but it is 

 equally certain that the thread may appear from the beginning 

 in a number of distinct segments, i.e. as a segmented spireme, 

 and there are some cases in which no distinct spireme can be 

 seen, the reticulum resolving itself directly into the chromosomes. 

 The chromosomes, when fully formed, vary greatly in appear- 

 ance. In many of the tissues of adult plants and animals they 

 are rod-shaped and are often bent in the middle like a V (Figs. 

 28, 131). They often have this form, too, in embryonic cells, as 

 in the segmentation-stages of the ^gg in Ascaris (Fig. 31) and 

 other forms. The rods may, however, be short and straight (seg- 

 menting eggs of echinoderms, etc.), and may be reduced to spheres, 

 as in the maturation-stages of the germ-cells. In the equatorial plate 

 the V-shaped chromosomes are placed with the apex of the V turned 

 toward the spindle (Fig. 28), while the straight rods are placed 

 with one end toward the spindle. In either case the daughter- 

 chromosomes first begin to move apart at the point nearest the 

 spindle, the separation proceeding" thence toward the free portion. 

 The V-shaped chromosomes, opening apart from the apex, thus give 

 rise in the early anaphase to < >-shaped figures ; while rod-shaped 

 chromosomes often produce x- and _L-shaped figures (the stem of the 

 X being double). The latter, opening farther apart, form straight 

 rods twice the length of the original chromosome (since each consists 

 of two daughter-chromosomes joined at one end). This rod finally 

 breaks across the middle, thus giving the deceptive appearance of a 

 transverse instead of a longitudinal division (Fig. 52). The <>- 

 shaped figures referred to above are nearly related to those that 

 occur in the so-called Jieterotypical mitosis. Under this name Flem- 

 ming {"i'J) first described a peculiar modification of the division of the 

 chromosomes that has since been shown to be of very great impor- 

 tance in the early history of the germ-cells, though it is not confined 

 to them. In this form the chromosomes split at an early period, but 

 the halves remain united bv their ends. Each double chromosome 

 then opens out to form a closed ring (Fig. 37), which by its mode of 

 origin is shown to represent two daughter-chromosomes, each forming 

 half of the ring, united by their ends. The ring finally breaks in two 

 to form two U-shaped chromosomes which diverge to opposite poles 



