THE CHROMOSOMES 99 



form of the chromosomes composing its genomes (Fig. 71). Crepis 

 capillaris has a genome of only 3 members, all of which differ in their 

 length and in kinetochore location, while one of them has a satellite. 

 In Datura, another genus prominent in c.ytogenetics, there are 12 members 

 (Fig. 72). The genome has 2 members in some fungi, 4 or 5 in some 

 species of Crepis, 7 in the garden pea, 8 in the onion, 9 in the cabbage, 

 10 in maize, 12 in many conifers, 19 in some willows, 2 or more in aphids, 

 6 in the house fly, 12 in various salamanders, 19 in the cat, 30 in the 

 cow, sheep, and horse, and 24 in man (Fig. 73) and the Rhesus monkey. 

 Somatic nuclei, of course, contain double these numbers. 



In a diploid chromosome complement the two members constituting 

 each pair of similar chromosomes are said to be homologous. They have 



e 



M 15 n 





Fig. 72. — The genome of Datura Fig. 73. — Diploid complement of 48 



stramonium frona a monoploid root tip. chromosomes from human spermato- 



The ends of the vaiious chromosomes are gonium. {After O. Minouchi and T Ohta.) 

 designated by arbitrary numbers. {After 

 S. Satina, D. Bergner, and A. F. Blakeslee.) 



an ultimate common origin and affect the same group of reactions in 

 the life of the organism. As a general rule the chromosomes of the 

 complement in a somatic cell may occupy any relative position in the 

 nucleus or the mitotic figure without respect to their homologies. In 

 exceptional cases, however, notably in Diptera, there is a strong tendency 

 for the homologues to lie rather near each other. This is well shown in the 

 ganglion cells and spermatogonia of Drosophila (Fig. 74). The somatic 

 complement of D. melanogaster has 8 members : two large V-shaped pairs, 

 one very minute pair, and one pair of sex chromosomes (XX in the female, 

 XF in the male). In the salivary glands and certain other larval tissues 

 the pairing becomes very intimate, each giant chromosome being in 

 reality two homologous members in close union. Such a condition, 

 which resembles the synapsis normally occurring in the meiotic jirophase, 

 is not known to occur elsewhere in somatic nuclei. This phenomenon is 

 exceedingly useful to the cytogeneticist, for it often makes it possible 

 for him to compare very minutely the chromosomes of two strains or 

 species after a cross has been made between them (page 184). It is 



