EXPERIMENTAL EMBRYOLOGY ■ 391 



Induced Parthenogenesis. In normal fertilization the sperm im- 

 parts not only the activating stimulus but also a set of genes em- 

 bodying the contribution of the male parent toward the make-up 

 of the new individual. The prime interest in induced parthenogene- 

 sis lies in the fact that if the stimulus can be provided without the 

 usually accompanying paternal genes, it would greatly facilitate the 

 task of the geneticist in producing a homozygous true-breeding 

 type, which otherwise requires a long and laborious process of self- 

 fertilization (East, 1930). 



Ever since the initial discovery of a Datura haploid (Blakeslee 

 etal., 1922) a variety of physical and chemical treatments have been 

 tried to achieve this result. The chief of these are (1) exposure to 

 very high or very low temperatures soon after pollination; (2) use 

 of X-rayed pollen on stigma; (3) use of foreign pollen or of delayed 

 pollination; and (4) chemical treatment. 



To give an exhaustive survey of the successes and failures that 

 have attended these efforts is beyond the scope of this book. A 

 few examples are mentioned, however, to indicate the nature of the 

 work that has been done. 8 



Concerning the effect of temperature, it is interesting to note that 

 Muntzing (1937) obtained a haploid plant of Secale cereale by ex- 

 posing the spikes to low temperatures (0.3°C.), 9 while Nordengkiold 

 (1939) achieved the same result by exposing them to high teupera- 

 tures (41 to 42°C.). 



Kihara and Katayama (1932) obtained three haploids of Triticum 

 monococcum from spikes which had been exposed to X-rays at the 

 time of meiosis. Later, Katayama (1934, 1935) pollinated the stig- 

 mas with X-rayed pollen, and out of 91 seedlings raised by him, 16 

 turned out to be haploids. In another strain, which normally pro- 

 duces about 0.5 per cent haploids, Kihara (1940) was able to increase 

 the percentage to 13.66 by using the same method. However, 

 other workers have been less successful, and Smith (1946) reports 

 that he failed to obtain any increase in the number of haploids by 

 X-ray treatment. 



The use of foreign pollen for inducing haploidy was first brought 



8 For further information on this topic, see reviews by Ivanov (1938) and Kostoff 

 (1941). 



9 Muntzing had really attempted to produce a doubling of chromosomes but 

 obtained instead a semilethal haploid. 



