1. A study has been made of the somatic chromosome
complex, the chiasma frequency and the relationship of sex to crossing-over and chiasma frequenc
in the male and female rat-Rattus norvegicus
2. The stages of mitosis in relation to the somatic
chromosomes have been discussed.
3. The number of chromosomes in the cells of both
males and females have been determined as 42.
which is in accord with the findings of Painter
(1926, 1927), Pincus (1927) and Swezy (1928).
4. Stages of meiosis have been followed, the nature
of pairing, formation of chiasmata etc. being
5. The chiasma frequencies have been calculated for
both the male and the female in diplotene and
metaphase, by interpreting the configurations in
the light of the partial chiasmatype hypothesis.
(Janssens 1;09, 1924 and ;Darlington 1931, 1932).
6. In addition, in the male where mitotic stages
were more abundant, the chiasma frequency has bee
calculated in the diakinesis stage as well.
7. It has been noticed that as meiosis progresses,
the frequency of chiasma formation decreases and
the terminal associations of the chromosomes
increases. The apparent movement of chiasmata
and the accompanying 'terminalisation' have been
S. The theories of Darlington and Sax have been
considered from the presentation of configurations in diplotene and metaphase stages.
9. The various types of bivalent found have been
interpreted, and the origin of various types in
metaphase, from configurations in diplotene, is
10. The precocious separation of some bivalents
accompanied by the tendency of others to lag has
11. Suggestions have been made of the relation
between genetical and cytological crossing -over
and a parallel his been drawn between them.
12. The genetical results of various workers on the
effect of sex on crossing -over has been summarised.
13. It has been found that in many forms there is a,
suppression of crossing -over in one sex-usually
14. The results recorded in this study of the
frequencies of chiasma formation, have been
compared with the genetical crossing-over results
15. In common with the genetical results in rodents
and in other examples, the frequency of chiasma
formation is lower in the heterozygous sex - which in the rat is the male.
16. The cause of the suppression of crossing -over
in the sex -heterozygote has been discussed and
the results of some other workers have been
17. The forms of sex- chromosomes commonly met with
have been listed.
18. The evolution of sex- chromosomes, the characters
carried by them, and the genetical aspect of sex
in individuals have been discussed.
19. The results given by Pincus (1927) have been
compared with those found in this study.
20. The types of bivalent found have been compared
and interoretatio is offered of some of Pincus's
21. The chiasma frequencies have been calculated
from each form and the results shown in