THE FUNDAMENTALS OF MENDELISM 39 



the normal or diploid number, or the tetraploid number (double the 

 diploid number). Boveri concluded that development could only go on 

 if there was one of each kind of chromosome, and that there were 

 the haploid number of different kinds each with a specific role in 

 development. 



Of the recent evidence on this point, the clearest perhaps is that 

 relating to organisms with one chromosome more than is normal for 

 the species (trisomies, p. 83). Thus in Datura stramonium the haploid 

 number is 12 and there are 12 kinds of primary trisomies known, each 

 with one extra chromosome which has a specific effect on the characters 

 of the plant. We shall see later that there are qualitative differences even 

 between different parts of the same chromosome. 



6b. Evidence that the chromosomes retain their identity 



The difficulty in proving that each chromosome retains its identity 

 through many cell divisions arises from the fact that its appearances 

 are only intermittent. As a rule no satisfactory demonstration of the 

 chromosomes in the interphase can be made. In life the interphase 

 nucleus appears optically homogeneous or granular, and on fixation it 

 shows either a granular structure, the graininess depending on the kind 

 of fixation employed, or a structure of anastomosing threads which are 

 swollen into nodes at the points of junction. Microdissection studies 

 suggest that the nucleus is homogeneous and liquid, and measurements 

 of viscosity give a surprisingly low value about equal to that of glycerine.^ 



There are some cases in which a resting phase is very short and the 

 chromosomes remain more or less distinct as separate vesicles through- 

 out it (premeiotic resting stage of spermatocytes, segmentation divisions 

 of eggs). In other cases parts of the chromosomes, particularly the 

 sex chromosomes (p. 78), may remain condensed and deeply staining 

 and thus distinguishable from the rest of the nucleus. Similarly, parts of 

 the chromosomes may remain condensed in the small nuclei of some 

 plants, when the diploid number of deeply staining bodies may exist 

 throughout the whole interphase; they are known as prochromosomes or 

 chromocentres. The connection of true nucleoli with these bodies and 

 with the rest of the nuclear apparatus is, in most organisms, imknown, 

 but in some plants (e.g. Vicia^ Zea) ^ and insects it has been shown that 

 the nucleoli arise after division in close connection with the unstaining 

 parts of certain chromosomes (the secondary constrictions; the chromo- 

 somes concerned have been called SAT chromosomes) and that when 



^ Cf. Gray 193 1, Heilbrunn 1928. ^ Heitz i93Ij McClintock I934- 



