130 



INTRODUCTION TO CYTOLOGY 



a large group is afforded by the Acrididse, a family of grasshoppers.^^ 

 It is stated that the degree of relationship here is as clearly expressed 

 in the chromosomes as in the externally visible characters, and that the 

 evidence indicates a descent by variation from a common ancestral 

 series of chromosomes paralleled by corresponding variation in somatic 

 structures (Robertson). 



Very often among plants and occasionally among animals the chromo- 

 some complements in a related group of individuals, races, or species 

 differ in the number of sets present, i.e., they form a "polyploid series." 



wm\ 



I -M 



H^ r EP <; p A 



^ F E p <: p A 



G H 



A P Q p e: F 



H 



nfii r 



Fig. 72. — Morphology of somatic chromosomes in several angiosperms. A, Gasteria 

 verrucosa. B, Crepis capillaris (virens). C, Cyrtanthus parviflorus: the six forms exhibited 

 by the eight pairs of chromosomes. D, Allium cepa: chromosomes in metaphase and 

 anaphase, showing two satellites in tandem ; seen in one plant. E, Haworthia cymbiformis. 

 F, Gasteria verrucosa. G, Aloe arborescens. Compare E, F, and G. (After W. R. Taylor, 

 1924, 1925a6.) 



Sometimes the chromosome numbers do not form a series of multiples. 

 Consideration of the origin and consequences of these conditions must 

 be postponed to later chapters. 



It is not to be doubted that such similarities and differences in 

 chromosome complements are of the greatest importance to students of 

 genetic relationships among organisms. At the same time caution is 

 necessary in the use of such evidence. It should not be assumed simply 

 because two chromosomes in different organisms look alike that they 

 correspond in function, nor that chromosomes with similar functions 

 should always show exact resemblance (see Fig. 71). Not only is it 



2« McClung (1905 et seq., 1923, 1924), W. R. B. Robertson (1916), Saez (1930). 



