400 Edmund B. Wilson. 



nucleoli" which in such forms as Anasa or Alydus conjugate to 

 form the small central chromosome of the first mitosis. The dif- 

 ferences between the two forms have already been pointed out. 

 Their resemblances are, however, no less obvious, namely, their 

 usual central position in the equatorial plate, small size, occasional 

 persistence as chromosome-nucleoli in the growth-period, and 

 their late conjugation. This comparison finds very definite support 

 in the conditions I have described in Nezara, where the idio- 

 chromosomes are of equal size and appear as two equal micro- 

 chromosomes in the spermatogonia. From the analogy of other 

 forms it is very probable that the more primitive and typical form 

 of synapsis is that between chromosomes of like size. It is there- 

 fore probable that such a condition as that observed in Nezara 

 is a less modified one than that in which the idiochromosomes are 

 unequal; and that the latter condition has arisen through a second- 

 ary morphological differentiation of two chromosomes that were 

 originally of equal size, and perhaps are represented by the two 

 equal microchromosomes that appear in the spermatogonia of 

 such Hemiptera as Anasa, Alydus, Syromastes or Protenor. 

 This comparison involves two assumptions, namely, first that in 

 case of the idiochromosomes the final conjugation of the micro- 

 chromosomes has been postponed from the prophases of the first 

 division to those of the second; and secondly that a reversal in the 

 order of the reduction- and equation-divisions has taken place in 

 case of these particular chromosomes, the first division being in 

 case of the Anasa-type the reduction- and in case of the idiochro- 

 mosomes the equation-division. The difficulty apparently involved 

 by the second assumption is less serious than may appear. All 

 the facts at our command indicate that a reduction-division 

 is the necessary, or at least invariable, sequel to a foregoing 

 conjugation; and if, as in the case of the idiochromosomes, the 

 final conjugation is deferred to the second division, the reduction- 

 division must also be deferred. The univalent idiochromosomes — 

 as is shown with certainty in case of the larger one in Lygaeus — 

 undergo longitudinal division at the same stage of the growth- 

 period as their bivalent companions and are already double 

 at the time of the first mitosis. There is, therefore, no difficulty 

 in the way of assuming — indeed, the facts seem to admit of no 

 other conclusion — that this is the equation-division. 



It must be recognized, however, that the foregoing comparison 



