204 MONTGOMERY A STUDY OF THE CHROMOSOMES 



they retain this position if they are of comparatively large size (as in most Pentatomidce) 

 but do not when they are of comparatively small size (as particularly in the Reduviidce). 

 Now the true nucleolus is rarely in contact with the nuclear membrane when it is not 

 apposed to a chromatiu nucleolus, so that when the two are mutually apposed it appears 

 to depend upon their relative volumes whether they will be peripheral or central iu 

 position a very large chromatiu nucleolus pulling to the periphery the true nucleolus, a 

 small chromatin nucleolus being pulled by the true nucleolus toward the centre of 

 the nucleus. 



That the chromatin nucleoli are morphologically chromosomes is shown particularly 

 in mitosis, when they simulate in form and divide like chromosomes; an examination of 

 the chromatin nucleoli, designated N. 2, in the plates of the present paper, demonstrate 

 this point. The univalent chromatin nucleoli of the spermatogonia generally unite to 

 form bivalent ones in the synapsis stage just as do the chromosomes, and generally the 

 number of them in the spermatocytes is just half of that in the spermatogonia (certain 

 exceptions shall be considered later). In the first maturation division each bivalent 

 chromatin nucleolus is halved transversely (reduction division) just as are the bivalent 

 chromosomes. 



Thus the chromatin nucleoli are essentially chromosomes, but chromosomes which 

 preserve a compact form and dense structure throughout the rest period. 



(b) Number and Valence. 



It is most frequently the case that there are two uuivalent chromatiu nucleoli in the 

 spermatogonia, as in the Pentatomidce, Euryyaster, Coreidce, Peliopelta, (Edancala, Phy- 

 mata, Coriscus (?), Hygotrechus, Zaitha. In these cases the two chromatin nucleoli join 

 together to form one bivalent one in the growth period ; but in Euchislnx tristigmus and 

 Chariesterus the two frequently remain separate in the growth period, yet this is not a 

 complete separation for there would seem to be a linin connection between the two, since 

 they generally become more closely approximated at the time of the first maturation 

 division when such a linin connection may be seen. But in some of these species (Peri- 

 balus, Coenus, Trichopepla, Coriscus) besides the bivalent one are found one or two (three 

 or four in Trichopepla) much smaller ones, which appear to be univalent, in the sperma- 

 tocytes ; whether these are represented in the spermatogonia or whether they arise in the 

 spermatocytes for the first time, I could not determine on account of their minuteness. 



In Oymus and Jchnodemus I could not determine the number in the spermatogonia, 

 but since here there is one bivalent one in the spermatocytes, there would probably be 

 two univaleut ones in the spermatogonia. The same would probably be true for Corizus 

 and Leptopterna. 



The Reduviidce and certain Capsidat show a larger number of chromatin nucleoli. 



