520 Edmund B. Wilson. 



in the initial anaphase, always separating in advance of the 

 larger chromosome-halves (Fig. 2, /). It is not possible in the 

 prophases just described to identify the heterotropic chromosome; 

 but from the analogy of Alydus, Syromastes and Archimerus it 

 may be assumed with great probability that it is the "odd or 

 eccentric chromosome which in the metaphase-group lies outside 

 the principal ring (Fig. 2, k). 



During the growth-period, as Paulmier described, the chromo- 

 somes, with the exception of the single conspicuous chromosome- 

 nucleolus, remain in a loose, diffused, lightly-staining condition 

 from the post-synaptic spireme stage until the condensation of the 

 tetrads begins; and until the end of this period the m-chromo- 

 somes cannot be distinguished. Throughout this whole period 

 the chromosome-nucleolus is distinctly visible; and it may at 

 every period, even in hematoxylin preparations, if long extracted, 

 be at once distinguished from the true nucleolus or plasmosome 

 (as is shown in Paulmier's figures), since the former stains intensely 

 black, the latter pale blue or in double-stained preparations, pale 

 red or yellow. In the contraction-phase of the synaptic period 

 it is more or less elongated, ovoidal, or sometimes slightly con- 

 stricted in the middle (Fig. 2, a). In the late post-synaptic 

 period, at a time when the other chromosomes are beginning to 

 shorten and to give rise to the characteristic double cross-figures 

 and V-figures it is usually more or less elongated, the transverse 

 constriction is less obvious or disappears from view, and the body 

 often shows faintly but distinctly a longitudinal split. (Cf. Paul- 

 mier, Fig. 22.) Slightly later, as the other chromosomes continue 

 to shorten and thicken, the chromosome-nucleolus also shortens 

 and thickens, often assuming a spheroidal form in which a central 

 cavity may sometimes be seen. As the remaining chromosomes 

 condense to form the tetrads it again alters its shape, often becom- 

 ing bipartite (Fig. 2, -</), but sometimes showing a more or less 

 distinctly quadripartite form as described by Paulmier (e. g. y in his 

 Figs. 23, 24). It now becomes indistinguishable from the other 

 larger chromosomes, since the latter have also condensed into 

 similar tetrads or dyad-like forms, but the two m-chromosomes are 

 immediately recognizable by their small size. It might therefore 

 be supposed that the chromosome-nucleolus has divided to form 

 the two microchromosomes, as Gross believed to be the case in 

 Syromastes. The stage that immediately precedes this gives, 



