162 ORGANISMS ILLUSTRATING BIOLOGICAL PRINCIPLES 



macronucleiis, elongates and divides twice in rapid succession. Three 

 of the micronuclei thus formed in each cell disappear, but the fourth 

 one divides again. In this last division two irregular masses of chro- 

 matin are formed. This process has been likened to a similar division 



inr 



"Xcr 



Conjugation in Paramecium caudafiim. Shortly after the conjugating pair 

 come together with their ventral surfaces opposed (I) a protoplasmic bridge is 

 formed, the macronucleus breaks down (II) and each micronudeus divides a 

 second time (III). What happens to three of the four micronuclei? Compare 

 this stage with the figure on page 429 (maturation). Next the micronuclei 

 remaining in the cell divide into two, the smaller (migratory) micronucleus passing 

 over by the protoplasmic bridge into the opposite cell, there to unite with the 

 larger (stationary) nucleus (VI). Trace the subsequent divisions of the fused 

 micronucleus (VII, IX). How do we get back to the original cell condition? 

 (X-XIV). (After Hegner.) 



that takes place in the eggs of animals, at the period known as matu- 

 ration, when the sex cells are losing part of their chromatic material 

 in preparation for fertilization of the egg by the sperm cell. The 

 smaller mass is thought to correspond to a sperm cell of the many- 

 celled animals, while the larger one corresponds to the egg cell. In 

 any event, each of the smaller micronuclei migrates reciprocally over 



