NATURE OF THE GENETIC EFFECTS 357 



also especially subject to having its frequency influenced by varied 

 conditions. 



The primary function of crossing over, as of recombination of genes in 

 different chromosomes, lies in its allowing advantageous mutant genes 

 that arose in different lines of ascent an opportunity to become associated 

 with one another in lines of descent that inherit both or all of these genes 

 at once, without the much longer delay that would usually be required 

 for both or all of them to arise by successive mutations within the same 

 line. In other words, evolution is accelerated by crossing over. In 

 addition, crossing over, tying the four chromatids of a tetrad together by 

 means of the cross figure (chiasma) formed at each point of breakage and 

 cross-union serves in most organisms to keep the chromatids from falling 

 apart into the separate pairs that would usually result from their tend- 

 ency to hold together mainly in twos; thus it facilitates their being so 

 oriented at the meiotic divisions as to segregate in a regular fashion, one 

 chromatid of each tetrad to each gamete. 



3. EFFECTS OF RADIATION ON THE TRANSMISSION 

 OF THE GENETIC MATERIAL 



3-1. Interference with Cell Division and Induction of Polyploidy. The 

 most immediately observable conspicuous effect of radiation on the trans- 

 mission of the genetic material is its inhibition of mitosis. This effect is 

 produced by all ionizing radiations as well as by ultraviolet, even in very 

 small doses, and it is a striking fact that powerful chemical mutagens such 

 as those of the mustard gas group also produce it. If a cell is already as 

 far along in mitosis as a late prophase, metaphase, anaphase, or telophase 

 stage when the radiation is applied, it will complete its division without 

 interruption; but, if approaching prophase, it will be inhibited from 

 entering this stage for a period of time that may be considerable, depend- 

 ing upon the material and the dose. If in an early or middle prophase it 

 may even appear to regress in phase and will then remain mitotically 

 static until finally cells that had been in interphase during treatment have 

 caught up with it. Thus there is a kind of damming up of mitoses, 

 followed by a burst of them. This crest in the mitotic frequency is in 

 turn succeeded by a trough, since many of the cells that otherwise would 

 have been dividing have then, because of the previous delay, only recently 

 entered interphase. The resulting tendency to synchronization of 

 mitoses is thereafter expressed in a series of gradually subsiding waves of 

 mitotic frequency. 



In the fertilization period of the eggs of certain echinoderms, E. E. Just 

 (1926) found that ultraviolet radiation interfered with separation of the 

 polar bodies. The four haploid nuclei then united with one another and 

 with the sperm nucleus to form a pentaploid nucleus, i.e., one with five 



