524 Cytogenetics and Evolution 



result from a disharmony within the embryo itself such that 

 the embryo is incapable of developing sufficiently normally to 

 live. On the other hand, it may sometimes result from a dishar- 

 mony between the developing embryo and some other structure 

 in the seed, such as the seed coat or the endosperm. If the dis- 

 harmony is within the embryo, apparently nothing can be done 

 about it. Otherwise, the embryo may be removed from the seed, 

 cultured in vitro, and thus raised to maturity. Although it can- 

 not occur in nature, it is an interesting method of producing hy- 

 brids artificially in a few plants. 



Hybrid Sterility Isolation. Another type of isolating mecha- 

 nism results in the formation of sexually mature hybrids between 

 the two parental species, but hybrids which are sterile. Such 

 sterility is often the result of failure of the chromosomes to pair 

 and to segregate normally at meiosis. It does not prevent the 

 formation of hybrids but of hybrid swarms. 



Combined Isolation. Although we have listed a number of 

 mechanisms that isolate species, it so happens that many species 

 are isolated simultaneously by a number of mechanisms. For 

 example, Dobzhansky has shown that although the hybrids be- 

 tween Drosophila miranda and D. pseudoobscura are completely 

 sterile, these species are also separated by a strong sexual (psy- 

 chological) isolation and by a decrease in the viability of the 

 Fi hybrids. Probably more species are isolated by the simul- 

 taneous operation of two or more mechanisms than by merely 

 one mechanism. 



Embryo Culture 



The culture in artificial media of young embryos of Datura 

 hybrids has been spectacularly successful during the last few 

 years as the result of a technique developed by van Overbeek 

 and Conklin. The hybrid embryos are dissected out of the seed 

 and are placed in media containing certain salts, vitamins, and 

 an "embryo factor" found in coconut milk. Blakeslee and his 

 co-workers have obtained hybrid embryos from eleven species 

 combinations that had never before been successful and from 

 other combinations that had previously yielded only one seed 

 from many hundred pollinations. One of the most interesting 

 hybrids thus obtained was between Datura inoxia and a tree 

 Datura. Blakeslee has varied the coconut milk technique by 



