Studies on Chromosomes. 389 



may occur one or more distinctly quadripartite forms (Fig. 6r). 

 The history of these dyad-hke bodies clearly shows, I think, that 

 with the exception of the idiochromosomes they are derived from 

 bivalent longitudinally split rods, and they have therefore the 

 same valence as the actual tetrads with which they are associated. 

 The nuclear membrane now disappears and the chromosomes are 

 drawn into the equatorial plate of the first mitosis. 



The general history of the growth-period in Ccenus is similar to 

 that of Lygaeus, but is in some respects abbreviated; and stage / 

 is much less marked, the chromosomes not losing their boundaries 

 or their staining-capacity in so great a degree, and still presenting 

 the appearance of a ragged interrupted spireme. 



(b) The Idiochromosomes during the Growth-period. 



The foregoing description applies only to the larger or ordinary 

 chromosomes. Throughout the whole of the growth-period in 

 Coenus and from stage e onward in Lygaeus, at least one of the 

 idiochromosomes can always be distinguished as a compact, 

 spheroidal, intensely staining chromosome-nucleolus, and fre- 

 quently both idiochromosomes are distinguishable in this form in 

 all of these stages. The early stages of Lygaeus (b to d) are of 

 especial interest in that the condensation of the idiochromosomes 

 is delayed, and at least the larger one still has the form of an elon- 

 gate, longitudinally split rod or thread. Even at this time, however, 

 it is immediately distinguishable from the others by its greater 

 thickness and greater staining capacity. It is clear beyond all 

 question, therefore, that at least the large idiochromosome may 

 retain its identity throughout the whole growth-period. With the 

 small idiochromosome the case is not so strong, as will be seen 

 from the foUowino- account. 



It is convenient to trace the history of the idiochromosomes in 

 the reverse order from stage i backward, again taking Lygaeus 

 as the type. In this stage (late prophase), when the six larger 

 chromosomes are in the form of condensed tetrads or dyad-like 

 bodies, both the idiochromosomes have very distinctly the form 

 of dyads. The nucleus now contains therefore eight separate 

 chromosomes, among which the idiochromosomes are at once 

 recognizable by their small size (6r). In stage h the idiochromo- 

 somes have the same general appearance, though their bipartite 



