88 



Messrs. C. Dobell and A. P. Jameson. 



of the chromosomes can be followed with comparative ease all through the 

 life-history. 



The gamont nucleus gives rise to a small achromatic spindle, on to the 

 equatorial plate of which three tiny chromosomes pass. These are formed 

 from a little vesicular karyosome which lies near the spindle, and which is 

 all that remains of the originally abundant chromatin of the nucleus. They 

 are at first filamentar, but shorten and thicken until they become globular on 

 the equatorial plate (see fig. 2, A). Each chromosome divides by constriction 

 into two at the metapha.se (fig. 2, B), and three daughter chromosomes pass 

 to each pole of the spindle (fig. 2, C). Of the three chromosomes two are 

 approximately equal in size and round in outline, while the third is slightly 

 larger and ovoid. 



In the second mitotic division the chromosomes are again present in the 

 form of three globules. At the third, however, they are stumpy rods. But 

 at the next division, and those immediately following it, they are filamentar. 

 They split longitudinally at the metaphases (fig. 2, D) and pass as filaments 

 to the poles of the spindles (fig. 2, E). A definite size-relation is usually 

 visible — one chromosome being long, one somewhat shorter, and the third 

 intermediate (cf. fig. 2, E). 



During later divisions the nuclei become smaller, and travel to the 

 periphery of the organism. At the same time the chromosomes become 

 shorter and more stumpy (fig. 2, F). The nuclei multiply still further at 

 the periphery. When they have become very numerous and small — 

 immediately previous to gamete formation — the chromosomes are seen to 

 be once more globular, and to divide by constriction (fig. 2, G-). The gametes 

 are now formed, and fuse in pairs. 



When the nucleus of the zygote (sporoblast) is about to divide, its 

 chromatin breaks up into numerous granules, which subsequently unite to 

 form a spireme. The spireme is at first an open one, but it afterwards 

 contracts into a somewhat tangled knot towards one side of the nucleus. 

 The knot then opens out, and the spireme segments into six filamentar 

 chromosomes (fig. 2, H). Of these two are long, two short, and two inter- 

 mediate. At the division which now takes place these separate into two 

 homologous sets of three each, which pass to the daughter nuclei (fig. 2, K). 

 The latter rapidly divide twice in succession, thus giving rise to the eight 

 sporozoite nuclei. Three chromosomes are visible at each of these divisions : 

 but at the second they are short rods (fig. 2, M), and at the third globular 

 (fig. 2, N). 



It will thus be seen that the number of chromosomes in all the nuclear 

 divisions of D. schneideri is, with one exception, three. At the first mitosis 



