126 INTRODUCTION TO CYTOLOGY 



of the diploid complement tend to show a very regular paired arrange- 

 ment (see Metz, 1916). Similar but less distinct pairing is sometimes 

 reported in plants. An especially interesting case is described by Robert- 

 son (1930) in Paratettix texanus, a grasshopper (Fig. 68). In individuals 

 produced by the union of two gametes in the usual w^ay the two parental 

 chromosome sets tend to remain distinct in the developing tissues, 

 whereas in individuals produced by parthenogenesis the two sets (evi- 

 dently formed here by chromosome division in the incomplete maturation 

 of the egg) have a distinctly paired arrangement. In both cases the 



configuration in the egg as it begins development is 

 maintained through the successive cell-divisions. 



All of these facts suggest that there are always 

 factors, some of them purely physical and some involv- 

 ing the chemical and genetic constitution of the 

 chromosomes, which tend to give the members of a 

 complement a certain arrangement, and that if these 

 factors were always given unrestricted opportunity 

 for action each type of complement might always 

 exhibit about the same characteristic configuration. 



Chromosome Complements of Related Organisms. 

 The discovery of the relative uniformity of the number 

 of chromosomes in an individual or a species led to the 

 5^ g determination of the number in many organisms 

 Fig. 68. — Chro- belonging to all of the major groups. A certain 

 uxZ^s.^ °A^iTomhi- amount of speculation regarding the general phylogen- 

 parentally produced etic significance of chromosome number has been 

 Iri'^'paTses*^^^^^^^^ indulged in, but students of the subject have reached 

 the parental sets. B, the conclusion that the data at hand do not warrant 

 ca°lS prfd^ucld^TiTdi- ^^^y broad generalizations. So far as the large 

 vidual. {After w. groups are Concerned, there appears to be no correla- 

 son, .) ^^^^ between taxonomic position and structural com- 



plexity, on the one hand, and the number, length, width, or volume of 

 the chromosomes, on the other. It is, however, generally agreed that 

 within restricted circles of affinity the number of chromosomes, and 

 especially their size and form, may often afford important evidence in 

 the determination of genetic relationships.^^ 



The type of chromosomal complex characteristic of any individual or 

 group of allied forms may be called its karyotype (Lewitsky, 1924). ^^ 



21 McClung (1908), Strasburger (19106), Farmer and Digby (1914), Tischler 

 (1916, 1921-1922), Winge (1917), Gates (19206, 1924a6), Marchal (1920), Meek 

 (1920), Heilborn (1924). See especially Lewitsky (1931c) for chromosome morphol- 

 ogy. See also footnotes, pp. 23-28. 



22 The term was originally used in a more restricted sense by Delaunay (1922). 

 See the discussion by Lewitsky (1931c). 



