THE CORRELATION OF SOMATIC CHARACTERS, ETC. 107 



Tlie Con-elation of Somatic Characters and Chromatin Rod-Lengths, being 

 a Further Study of Chromosome Dimensions. By C F. U. Meek, 

 M.Sc, F.L.S., F.Z.S. 



(With 5 Text-figures.) 

 [Read 20th June, 1912.] 



Introduction. 



In a recent paper * dealing with chromosome dimensions in numerous 

 organisms, I have shown that throughout the animal kingdom lengths of 

 component rods appear to constitute members of a general series in arith- 

 metical progression, whereas only three diameters exist, viz. "21 ^ in 

 ■Protozoa, and •12 and "83 fi in low and higher Metazoa respectively. 



( Consideration of the results given has, moreover, led to the enunciation of 

 an hypothesis which postulates a series of cycles in the course of phylogeny. 

 It is suggested that the chromatin granules of the simplest Protozoa have 

 been converted into rods by purely linear growth, accompanying evolutionary 

 development and increasing somatic complexit}-, and, since the rate of this 

 orowtli cannot have been the same in all chromosomes, rods of various lengths 

 have been evolved ; examples of such complexes can be seen in Ciliata and 

 other highly differentiated Protozoa. A stage in phylogeny was later reached 

 when a maximum rod-length had been attained, such limit having been 

 imposed by spindle mechanism or other physical conditions ; when this 

 occurred chromatin units conjugated in fours, and the normal thread-width 

 was thus doubled. The chromosomes, reduced in number, then segmented 

 transversely into numerous spheres of the new diameter, and the process, 

 which approximately re-established the number of chromosomes previously 

 seen, enabled them to enter a fresh course of Knear growth accompanying- 

 further evolutionary dejfelopment. In this manner the complexes of low 

 Metazoa may have evolved from those of Protozoan ancestors. 

 ■ When the length-limit of chromosomes was again reached, conjugation of 

 units once more occurred, and this w^as followed as before by segmentation 

 into spheres of the new diameter ; the last named having been thus doubled, 

 became identical with that now found in organisms belonging to phyla above 

 and including Nemathelminthia. Thus the chromatin thread-width of the 

 high Metazoa may have evolved from that of the lower. 



This hypothesis seems to accord with phenomena, for I have been able to 

 iind in the animal kingdom examples that apparently represent stages of 



* "A Metrical Analysis of Chromosome Complexes," Phil. Trans. B,oj. Soc. ser. B, 

 vol. 203, 1912. 



