524 Edmund B. Wilson. 



positively to identify the heterotropic chromosome by its size; 

 though it is evidently not one of the largest ones, since the latter 

 form a symmetrical pair (Fig. i, k) which doubtless unite to form 

 the single macrochromosome of the spermatocyte-divisions (in 

 accordance with Montgomery's account of several other forms). 

 In Anasa, however, it may be regarded as highly probable that 

 the heterotropic chromosome is one of the largest three chro- 

 mosomes, the remaining two of which pair as usual to form the 

 spermatocyte macrochromosome-bivalent (Fig. 2, o, p). This is 

 confirmed by comparison with the chromosome-nucleolus at the 

 synaptic contraction-period (Fig. 2, a). At this time it varies 

 considerably in form, but is always more or less elongate, often 

 ovoidal, sometimes almost rod-shaped, and sometimes more or 

 less distinctly constricted in the middle; it rarely appears to be 

 composed of two symmetrical halves (described by Gross as the 

 typical condition in Syromastes.) It is rarely attached to a 

 plasmosome, the latter body, when present, being usually separate 

 (as in Fig. 2, a). 



The discrepancy in size between the chromosome-nucleolus 

 and the spermatogonia! microchromosomes is here still greater 

 than in Alydus. On the other hand, as a comparison of the 

 figures will show, the chromosome-nucleolus of this period is of 

 very nearly the same volume as one of the largest three spermat- 

 ogonia! chromosomes. All the facts therefore point to the con- 

 clusion that one of the latter is the heterotropic chromosome, 

 and that it persists throughout the growth-period as the chromo- 

 some-nucleolus, precisely as in Alydus or Protenor. Exactly the 

 same result is indicated in Archimerus, where the discrepancy in 

 size between m-chromosomes and heterotropic chromosome is 

 even greater than in Anasa (Fig. 3, ^, /). 



3. BEHAVIOR OF THE HETEROTROPIC CHROMOSOME IN THE 

 MATURATION-DIVISIONS OF ARCHIMERUS CALCARATOR. 



In all the Hemiptera thus far described (Pyrrochoris, Anasa, 

 Alydus, Protenor, Syromastes, Harmostes, CEdancala, Charies- 

 terus), the heterotropic chromosome, when present, divides equally 

 in the first spermatocyte-mitosis, but fails to divide in the second, 

 thus showing a marked contrast to the phenomena in the Orthop- 

 tera where the reverse order occurs. In the present section I wish 



