42 The Chromatin in the Development of Hybrids 



lished the first research directed specifically toward the solution of this 

 question. Riickert found, in the first place, that the condition of double- 

 nuclei could be followed considerably beyond the late cleavage stages and, 

 in the second place, that the chromosomes might be arranged in two 

 groups upon the spindle, more or less distinctly separated. From the 

 bilobed nuclei of the one, two, four, etc. cells a double group of chromo- 

 somes might arise and these two groups could be followed each into one 

 of the two halves of the subsequent resting nucleus. Such bilateral 

 grouping of the chromatin in the spindle occurred only in the earlier 

 cleavage but the double nuclei could be found, although in constantly de- 

 creasing number, in later stages. The strong probability that in the 

 early stages the two halves of the double nuclei represent the double 

 source of the chromatin, makes the assumption that in the later stages 

 such double nuclei have a similar significance, justifiable. It is worth 

 while to state in this connection Euckert's cautious conclusions in his 

 own words, " Jedenfalls geht aus der vorstehenden Untersuchung her- 

 vor, dass in der ersten Entwickelungszeit mindestens bei einem Theile 

 der Kerne eine Vermengung der vaterlichen und miitterlichen Halfte 

 nicht statt hat, das ein soldier Vorgang fiir den normalen Verlauf der 

 Entwickelung somit nicht erforderlich ist. Das Chromatin kann seine 

 urspriingliche Vertheilung beibehalten trotz wiederholter mitotischer 

 Theilungen und Aufiosungen in ein feinfadiges Geriist, und obwohl die 

 iibrigen Lebensvorgange innerhalb seiner Substanz, die Assimilation . und 

 das Wachsthum, gerade zu dieser Zeit der rasch aufeinanderfolgenden 

 Theilungen lebhaftere sind als sonst." 



V. Hacker, 95, 02, Avorking also on Cyclops, carries the observations 

 of Euckert considerably further. In addition to the double nuclei and 

 the bilateral distribution of the chromatin on the spindle, he observed a 

 physiological difference in the maternal and paternal chromatin masses. 

 This physiological difference showed itself in the different stages in which 

 the two masses of chromatin may be within the same cell. It enabled 

 him often to distinguish two groups when otherwise there was no spatial 

 separation or no nuclear membrane to separate them. In Cyclops brevi- 

 cornis, however, he could not recognize the double distribution of the 

 chromatin beyond the eight-cell stage except in the primordial germ cells 

 from the beginning of gastrulation. 



Eecently, Conklin, 01, has shoMm that in Crepidula, even more clearly 

 than in Cyclops, the double character of the nuclei during certain phases 

 quite commonly obtains during earlier cleavage (first 5 or 6 generations), 

 and gives this the same interpretation that others had given it. 



Conklin called attention to the fact that in these double nuclei each 



