OF THE GERM CELLS OF METAZOA. 



211 



remains quite constant, each univalent chromosome being generally slightly elongate. 

 Thus the form is not wholly dependent upon the number, but particular groups <>i 

 Metazoa appeal' characterized by particular forms of chromosomes, as e. g. the Mollusca 

 with their rod-shaped chromosomes. Then the form of bivalent chromosomes is 

 dependent upon the mode of junction of the component univalent elements. In no 

 sense, however, can the form of the chromosomes be said to determine their number. 



So Par our considerations lead to only negative results; centrosomes and achromatic 

 spindle structures, nucleoli, absolute and relative maws of nucleus and cytoplasm, cell form 

 and chromosomal form seem not to be (actors determining the number of chromosomes. 

 As I attempted In show in a preceding [taper on Peripatus (1901), the chromosomes must 

 be regarded as individuals of a lower grade than what. I termed the "nuclear element," 

 namely, the linin spirem with the chromosomes arranged upon it. The problem is really, 

 then, why does this nuclear element show in one species a certain number of chromatin 

 segregations, in another species a different number? The more recent chemico-physio- 

 logical studies would lend to show that the chromosomes are centres of metabolic activity, 

 and accordingly the problem of the factors governing the chromosomal number maybe 

 closely connected with the phenomena of metabolism ; the number of the chromosomes 

 maybe dependent upon the nature! of the metabolism, as upon either the chemical nature 

 of the chromosomes themselves or upon that of the cell nutriment. The latter might be 

 experimentally tested by changing the food of a, species, and observing whether differ- 

 ences in the cumber of the chromosomes might thereby be obtained. But to conclude! 

 that (he number of the chromosomes is dependent upon the nature of the metabolism 

 -Iocs not solve the problem but only slates it mores precisely. 



Another question which arises in this connection is whether a, small or a, large; 

 number of chromosomes is to be; regarded as the primitive condition. A priori it would 

 appear probable that at an early phyletic period the number of chromosome's was not 

 lixe'd for the species but variable, and that by a process of natural selection the' number 

 gradually became fixed. But as a specie's gradually changes info another form the num- 

 ber of chromosomes may alse> be' changed, as will be shown in the next section, so that 

 we may speak of an evolution of the; number of chromosomes. On the principle of the 

 law of greater condensation of organs in progressing evolution, it might be; that a, large 

 number of chromosomes represents a more primitive condition than a, smaller number. 

 Within such a group. as the Hemiptera heteroptera, for instance, forms like the' Belosto- 

 matidce, Reduvndce, Gapsidce and Phymatidce would be primitive! in possessing a Larger 

 number of chromosomes, while the Pentatomddce, Seutellariidm and Lygmdm in possess- 

 ing a, smaller number should be; regarded as more specialized — more highly developed. 

 From sue-.h a, standpoint as this, the chromosomal number would be of taxonomic 



