310 GARY N. CALKINS 



The new macronucleus is formed as a product of the second 

 division of one nucleus resulting from the first division of the 

 fertilization nucleus. This always occurs prior to separation of 

 the two conjugants. The division starts in an equipolar mitotic 

 nucleus, but in the telophase there is a marked difference in the 

 two poles (figs. 83 to 85) . One nucleus, which is ultimately 

 absorbed, becomes a small, dense, homogeneous body of the typical 

 micronucleus fonn 2 ^ in diameter; the other nucleus, 5 m in diam- 

 eter, becomes vesicular with chromatin granules distributed about 

 the periphery and with a linin network filling the interior (fig. 85) . 

 Granules of chromatin appear on the reticulum at a later state 

 and the peripheral granules disappear. These internal granules 

 have only a weak staining capacity, so that the young macro- 

 nucleus at this period appears as a pale spherical body in striking 

 contrast to the intensely stained chromatin of the old macro- 

 nuclei and the new micronucleus. This is the condition of the 

 new macronucleus at the period of separation of the conjugants, 

 and its further history will be followed in connection with the 

 changes which the ex-conjugants undergo (section IV). 



B. History of the micronuclei 



The variations in the number of micronuclei in vegetative 

 stages make it difficult to determine the normal number that 

 are active during conjugation. Careful study of more than 500 

 pairs convinces me that the number is inconstant, even during 

 the important maturation stages. In order to determine if there 

 is a constant difference in nuclei in the two individuals of a pair 

 and in the early stages of conjugation, I made a careful count 



Fig. 27 Conjugating specimens each with two parachute nuclei and three 

 degenerating micronuclei. X 800. 



Figs. 28 to 44 Stages in the transformation of the parachute nucleus and for- 

 mation of the first maturation spindle. X 3200. Figures 28 to 34 show the centro- 

 somes and connecting fibers, the homogeneous nucleus and its metamorphosis 

 into, first, a chromatin network, and second, free granules of chromatin. Figures 

 35 to 39 represent the first type of maturation spindle with 24± chromosomes; 

 Figures 38 to 39 telophase of same. Figure 40, second type of first maturation 

 spindle with eight chromosomes; Figures 41 and 42, 43 and 44, anaphase and 

 telophase of same. 



