William J. Moenkhaus ^ 43 



half -11811311}' contained one nucleolus, so that these might also be re- 

 garded as maternal and paternal. Hacker, 02, in a recent preliminary, 

 summarizes the results of his comparative study of species of Cyclops. 

 He endeavors to show that the maternal and paternal chromatin masses 

 are each represented by a nucleolus in certain phases of the cell cycle. 

 Taking these as an index he is able to establish the individuality of the 

 parental chromatin masses throughout the cycle of an individual. 



Among plants Miss Ferguson, 01, has shown that in Pinus strobus 

 the male and female chromatin remains distinct during the first two 

 cleavages, as far as she has followed them, and suspects from sections 

 of later stages that this individuality may persist. 



The two important papers by Herla, 93, and Zoja, 95, on Ascaris 

 hybrids treat the subject from a standpoint so similar to that of my own 

 work that it will be advantageous to consider them later. 



The above brief survey of the work thus far done on this subject 

 enables me to point out the following facts: (1) The evidence upon 

 which the authors base their conclusions rests on the assumption that 

 the two halves of the double nuclei occurring beyond the two-cell stage 

 represent, the one, the mat-ernal, and the other, the paternal chromatin. 

 (2) The chromatin of the two parents not only retains its individuality 

 but also remains spatially separated or bilaterally distributed in the 

 nucleus during the various phases of division. In the following detailed 

 consideration of my own results I shall have occasion repeatedly to refer 

 to these facts. 



5. Conjugation of Pronuclei and the First Cleavage. — The time elaps- 

 ing between the moment of entrance and the time of conjugation of 

 the pronuclei is the same as that for the normal eggs. Thus, in the 

 Fundulus cross, at a temperature at which the first cleavage furrow 

 forms just two hours after impregnation, the male pronucleus has be- 

 come apposed to the female pronucleus at 55 minutes after fertilization. 

 At 65 minutes, they have usually become well fused. During its migra- 

 tion the sperm has become metamorphosed into a vesicle which cannot 

 be told from the female pronucleus. I have taken much pains to find 

 some distinguishing mark between the maternal and paternal chromatin 

 at this stage, hoping such might serve in distinguishing them in subse- 

 quent resting stages. The size, form and arrangement of the chromatin 

 granules in the two pronuclei, however, are, so far as I have been able 

 to make out, altogether similar. 



■ In a late metaphase of a Fundulus hybrid egg, 73 minutes after fertil- 

 ization, two chromatin masses can readily be made out (Fig. 16, Plate 

 II). The one is evidently made up of the long chromosomes which, in 



