CHROMOSOME ABERRATIONS IN ANIMALS 643 



development was inhibited by X-ray treatment; and White (1935a, b) 

 and Carlson (1941b) reported the induction by X rays in Orthoptera of 

 tetraploid spermatogonial cells containing diplochromosomes (tetraploid 

 with respect to chromatids but only double with respect to centromeres). 

 It is suggested that they may have originated in prophase cells that 

 reverted in phase at the time of treatment, and whose chromosomes 

 underwent a second doubling as they progressed toward metaphase the 

 second time. 



Individuals of a species in which both sexes are normally diploid may 

 occasionally have one, three, four, or more sets of chromosomes. For 

 example, Fankhauser (1945) noted the occurrence of haploid, triploid, 

 tetraploid, and pentaploid individuals among normal embryos raised 

 from eggs of diploid salamanders. Triploid females of D. melanogaster 

 arise spontaneously in normal diploid lines (Morgan, Bridges, and 

 Sturtevant, 1925). Deviations in the number of sets of chromosomes 

 may also be induced by ionizing radiations. Thus, in the classic experi- 

 ments of G. Hertwig (1911, 1927), P. Hertwig (1916, 1924), and Dalcq 

 and Simon (1932), exposure to radiations inactivated the nuclei of 

 amphibian eggs, which upon insemination with untreated spermatozoa 

 produced haploid embryos (androgenesis). In other experiments (cf. 

 Hertwig, 1911. 1913; G. Hertwig, 1927; and Dalcq and Simon, 1932), 

 treatment of spermatozoa with doses of radiation that did not impair their 

 motility or ability to penetrate the egg altered the subsequent behavior of 

 their chromosomes so that haploid embryos developed with only egg 

 chromosomes (gynogenesis) . In more recent experiments by Rugh 

 (1939), treatment of spermatozoa of Rana pipiens with doses of X rays 

 ranging from 15-10,000 r caused progressive decrease in the frequency of 

 viable embryos; at 10,000 r only 1.6 per cent hatched. With further 

 increase in dosage, however, the number of viable embryos increased, so 

 that at 50,000 r about 90 per cent hatched. These embryos were pre- 

 sumably gynogenetic haploids (see also Rugh and Exner, 1940). 



The method of inactivating a gamete nucleus by irradiation has been 

 used by A. R. Whiting (1946) to secure androgenetic males in the wasp 

 Habrobracon juglandis. Diploid males, which are nearly always sterile, 

 can be produced experimentally in this species, although the fertile males 

 are normally gynogenetic and haploid. Whiting irradiated females of an 

 inbred wild-type stock, whose egg chromosomes were in the metaphase 

 stage of the first meiotic division, with doses of X rays (up to 42,000 r) ; 

 they were then mated with untreated males carrying recessive marking 

 genes. Among the surviving progeny were fertile males showing the 

 characters of the recessive mutants (haploid and paternal in origin), in 

 addition to the expected wild-type males (haploid and maternal) and 

 females (diploid and biparental). Cytologic examination (A. R. Whiting 

 1948) indicated that the treatment retards and distorts the egg pronucleus 



