SUTTON 



words "Such phenomena may perhaps 

 be regarded as fulfilling the conception 

 of Strasburger and Boveri, that fer- 

 tilization may consist of two distinct 

 operations, the stimulus to develop- 

 ment and the union of characters in 

 the zygote." ^^ Division of the egg 

 without fusion of the pronuclei is a 

 well-known phenomenon having been 

 observed in eggs treated with chloral 

 (Hertwig brothers) or ether (Wil- 

 son) and may be supposed to occur 

 under certain unusual conditions in 

 nature. In the experiments mentioned, 

 however, both pronuclei continue to 

 divide separately, while for a cytologi- 

 cal explanation of the occurrence of 

 "false hybrids" it is necessary to con- 

 ceive not only the failure of the nu- 

 clei to copulate but the entire disap- 

 pearance of one of them. Such a case 

 would be comparable to that of chem- 

 ically induced parthenogenesis or to 

 the fertilization of enucleate egg-frag- 

 ments, according as the nucleus re- 

 maining was maternal or paternal. 

 Speculation in this connection, how- 

 ever, is unprofitable excepting so far 

 as it may serve as a guide to research. 

 A careful study of the fertilization of 

 such cases as Millardet's strawberries, 

 de Vries's Oenothera and Bateson's 

 Matthiola crosses will no doubt be 

 productive of immediate and positive 

 results. 



• Mosaics.— A fourth class of non- 

 Mendelian cases, the "mosiacs" or "pie- 

 balds" constitute a group in relation to 

 which, as I believe, only negative evi- 

 dence is to be expected from direct 

 cytological study. A good example of 

 the class is the "mosaic" fruit of Da- 

 tura obtained by Bateson and Saun- 

 ders, which, although in general ex- 

 hibiting the thornless recessive condi- 

 tion, showed in exceptional cases a 

 thorny patch. Of this case Bateson 



37 



says: "Unless this is an original sport 

 on the part of the individual, such a 

 phenomenon may be taken as indicat- 

 ing that the germ-cells may also have 

 been mosaic." I must confess my fail- 

 ure to comprehend just what is here 

 meant by mosaic germ-cells. I have 

 attempted to show that in all probabil- 

 itv^ the germ-cells are normally a mo- 

 saic of maternal and paternal chromo- 

 somes, but very evidently this is not 

 Bateson's meaning. 



From the standpoint of the chromo- 

 some theory I would suggest a possible 

 explanation of the conditions as fol- 

 lows: We have already assumed that 

 the somatic chromosome group, hav- 

 ing a similar number of members to 

 that of the cleavage nucleus and de- 

 rived from it by equation divisions, is 

 made up in the same way of pairs of 

 homologous chromosomes. Every so- 

 matic cell, by this conception, must 

 contain a double basis in the field of 

 each character it is capable of express- 

 ing. In strictly Mendelian cases one of 

 the homologues is uniformly dominant 

 throughout the parts of the organism 

 in which the character is exhibited. As 

 already noted, however, it is unlikely 

 that all the descendants of a dominant 

 chromatin entity will be dominant. 

 This is shown by the experiment of de 

 Vries with sugar beets, which are nor- 

 mally biennial but always produce a 

 small percentage of annual plants or 

 "runners," which latter are regarded 

 as recessives. The percentage of these 

 runners may be increased by rearing 

 the plants under unfavorable condi- 

 tions and this is taken as evidence that 

 the recessive allelomorphs may be- 

 come dominant under such condi- 

 tions.^^ 



If each cell contains maternal and 

 paternal potentialities in regard to each 

 character, and if dominance is not a 



19 Bateson and Saunders, loc. cit., p. 154. 



20 Cf. Bateson and Saunders, pp. 135, 136. 



