604 MARCUS M. HAETOG. 



gamete; Eg, it reaches and touches the female pronucleus; 

 E^, the male and female pronuclei fuse completely to form the 

 copulation-nucleus. 



Stage H is divided into the following phases : Hj, the off- 

 spring of the copulation-nucleus are all similar; H^, they are 

 diflFerentiated into enlarging meganuclei and small micro- 

 nuclei ; Hg, the meganuclei still enlarge, but refuse nearly or 

 altogether to take up stains ; H^, completion of the growth of 

 the meganuclei, which now stain well ; the exconjugate is now 

 ready to undergo bipartition. 



The separation of the exconjugates usually takes place in 

 stages F and H. I have tabulated the cases given by Maupas, 

 and find that in one species they separate in E, four in F, three 

 in G, eight in H; while in one species, Spirostomum teres, 

 they may separate in any stage from Fg to H^. 



For the sake of additional clearness, I have used the follow- 

 ing notation, ju or v = the micronucleus of a gamete ; Z, the 

 copulation-nucleus formed by the fusion of the two pronuclei. 

 M, the mega-, and m, the micro-nuclei of the exconjugate. To 

 denote the daughter nuclei after any given number of mitoses 

 I use this same number as an index to the letter of the original 

 nucleus ; thus Z^ (abbreviated for Z -f- 2^) represents one of the 

 two nuclei formed by the first mitosis of the copulation- 

 nucleus ; fx^ is one of the nuclei resulting from the third 

 mitosis of the original micronucleus of a gamete. 



We may now describe fully the process in one of the sim- 

 plest forms, Colpidium Colpoda, which has a single micro- 

 nucleus. This enlarges and divides twice in stages B and C ; of 

 the four nuclei so formed, three abort and are finally absorbed 

 by the cytoplasm, as '^corpuscules de rebut; " the fourth under- 

 goes a new bipartition (stage D) to form the two pronuclei 

 of the formula fx^. The point that determines which of the 

 four nuclei, jx^, should be preserved for bipartition seems to be 

 the accident of position; it is always the one at the 

 frontal end of the animal. 



So, too, position determines the respective fates of the 

 two pronuclei ; that which is next the point of union of the 



