314 CHROMOSOMES IN HEREDITY : PHYSIOLOGICAL 



still remain exceptions that do not agree with any of the rules 

 described. 



First, there are cases of precocious condensation of the third 

 type that seem to depend on hybridity or the absence of a partner 

 at zygotene. Such are the precocity of certain chromosomes in 

 hybrid roses (Erlanson, 1929) asynaptic Avena (Huskins and Heame, 

 1933) and Saccharum (Bremer, 1930), and in the fragments of 

 Camnula, an orthopteran (Carroll, 1920). It may be that these 

 represent the second type of differential condensation operating in 

 hybrids. 



Secondly, there are the B chromosomes of maize which are 



Fig. 102. — Metaphase plate of Zea Mays showing three condensed 

 B chromosomes. No. 6 is the nucleolar chromosome. X 5000. 



distinguishable by their behaviour at all stages of the life cycle. 

 Thus they are over-stained at metaphase of meiosis (Randolph, 

 1928) and at metaphase of mitosis. They show reduced 

 primary attraction at meiosis (McClintock, 1933) and probably 

 reduced secondary attraction at mitosis, where they never show 

 somatic pairing (Fig. 102). They are physiologically inert, and this 

 property may be related with their reduced attraction or reduced 

 specificity in attraction. It is difficult to know in what category 

 to place them or indeed the condensed chromosomes of the hepatic 

 Frullania, which show over-condensation equally at mitosis and 

 meiosis. 



Although it is not possible to put the whole of these hetero- 



