90 



Mr. E. P. Gregory. 



[-Jan. 5, 



He showed in 1892* that in Cyclops, as in Ascaris and other forms, 

 the two germ nuclei do not fuse in fertilisation, but give rise to two 

 separate groups of chromosomes which lie side by side in the spindles of 

 the dividing nuclei. Riickertf was able to trace the paternal and 

 maternal groups into the later stages of cleavage. Both observers 

 have shown that the two distinct groups of chromosomes appear also 

 in the germinal vesicle.J 



Hacker§ has since traced the autonomy of the paternal and maternal 

 chromatin in Cyclops from fertilisation up to the formation of the mother- 

 cells of the gametes ; and the double structure of the nuclei at the 

 maturation divisions is such that he is able to show "das die Vierer- 

 gruppen auf der einen Seite cler Scheidewand vaterlichen, auf der 

 anderen miitterlichen Ursprungs sind" (p. 341). In the "secundaren 

 Keimblaschen " the paternal and maternal chromosomes pass between 

 one another "in einer ganz gesetzmassigen Quadrillen-ahnlichen 

 Ordnung " (p. 342), with the final result that " Wahrend der Eireifung 

 von Cyclops findet eine Umordnung der Chroma tinelemente in der 

 Weise statt, dass die Eizelle in gleichmassiger Mischung grossvaterliche 

 mid grossmiitteiiiche Elemente erhalt " (p. 374). 



The regularity observed by Hacker in the movements of the 

 chromosomes at the reduction division, leading as it does to a sym- 

 metrical distribution of the chromosomes, may be our first indication 

 of a more comprehensive symmetry which probably underlies the pro- 

 duction of the different types of gametes in Mendelian hybrids. " It 

 is impossible to be presented with the fact that in Mendelian cases the 

 cross-bred produces on an average equal numbers of gametes of each 

 kind, that is to say, a symmetrical result, without suspecting that this 

 fact must correspond with some symmetrical figure of distribution of the 

 gametes in the cell divisions by which they are produced" (Bateson)..|| 



On the hypothesis that the segregation of characters occurs at the 

 reduction division, we shall expect that the mitoses in a Mendelian 

 hybrid will be perfectly regular, and in our present condition of 

 inability to recognise qualitative differences between chromosomes 

 alike in form, we should further expect that the mitoses will differ in 

 no visible way from those of the pure paternal and maternal races. 

 Cannon^l has shown this to be the case in race-hybrids of Pisum 



* "Die Eibildung bei Cyclops und Canthocampus," ' Zool. Jahr.,' 1892, vol. 5. 



* ; ' Ueber des Selbstandigbleiben der vaterlichen u. mutterlichen Kernsubstanz, 

 etc.," ' Arch. f. Mikr. Anat.,' 1895, vol. 45. 



X See Wilson, ' The Cell,' 2nd edit., 1902, p. 290 



§ " Ueber das Schicksal der elterlicben u. grosselterliclien Kernanteile," 'Jena 

 Zeitschr.,' 1903, vol. 37, p. 297 ; a review in ' Zool. Zentralbl.,' 1903, Jahrg. 10, 

 No. 11, p. 365. 



j| ' Mendel's Principles of Heredity,' Cambridge, 1902, p. 30. 

 % "The Spermatogenesis of Hybrid Peas," 'Bull. Torrey Bot. Club,' 1903, 

 vol. 30, p. 519. 



