Studies on Chromosomes. 513 



bivalent chromosomes, which show very marked and characteristic 

 size-difFerences (Fig. i, c-g, h). There are always (i) a largest 

 chromosome or macrochromosome, which is frequently quad- 

 ripartite; (2) a second largest; (3) three slightly smaller ones of 

 nearly equal size; (4) a fourth, considerably smaller than the last; 

 and finally (5) the smallest or michrochromosome-bivalent. 

 These show a characteristic grouping, the five larger ones forming 

 an irregular ring with the small bivalent ("chromatin nucleolus") 

 at its center, while the next smallest lies more or less at one side 

 of the ring (Fig. i, g). In the first division all these chromosomes 

 are equally halved (Fig. i, /). In the second all are again halved 

 with the exception of the second smallest which passes undivided 

 to one pole of the spindle (Fig. i, i-o). The size-relations leave 

 not the least doubt that this chromosome is derived from the one 

 of corresponding size in the first division — /. ^., the odd or eccen- 

 tric one — and the latter accordingly is to be identified as the 

 "accessory" or heterotropic chromosome. In the first division 

 this chromosome sometimes shows a quadripartite form (as was 

 described by Paulmier in Anasa) sometimes a dumbbell-shaped 

 or dyad-like form. In the second it is usually unconstricted and 

 often curved (Fig. i, /, m, w), sometimes into a U-shape so as 

 almost to appear double (Fig. i, o). 



A study of the growth-period shows that the heterotropic 

 chromosome may be traced uninterruptedly backward from the 

 metaphase of the first division to the contraction-phase of the 

 synaptic period, being always in the form of a condensed chro- 

 mosome-nucleolus, which in the early growth-period is attached 

 to a large, pale plasmosome, from which it afterwards separates. 

 It is impossible to mistake this chromosome, owing to the fact 

 that its characteristic size does not noticeably change except that 

 it becomes slightly larger as the growth-period advances (probably 

 owing to the presence of a central cavity), again becoming slightly 

 smaller as the general condensation takes place. (Cf. Fig. i, 

 a-c.) In the contraction-phase (Fig. i, a) and in the early post- 

 synaptic spireme the m-chromosomes are not visible, but as the 

 larger chromosomes assume the peculiar pale, ragged, clumped 

 condition, characteristic of the middle and late growth-periods, 

 the w-chromosomes frequently come into view, in the form of 

 two compact, intensely-staining bodies, that may occupy any 

 relative position (Fig. i, b). The period at which these bodies 



