INHERITANCE 761 



same type as one or the other of the two parents shows that the consti- 

 tution of the two parents is one important factor in determining the 

 type of the descendants. But why in the same cross the descendants from 

 some pairs should be of the same type as one parent, those from other 

 pairs of the same type as the other parent, while a few are of different 

 type from either parent — this is as yet quite unknown. 



In extremely rare cases the four clones produced by a particular pair 

 are not all of the same type. Thus in a certain case a pair composed of the 

 types A X C yielded four clones of the types A, B, B, B. In another case 

 A X C gave B, D, D, D. The great rarity of such cases indicates that such 

 results are due to irregularities in the cytological processes, comparable 

 to cases of non-disjunction of chromosomes. 



Immaturity and partial maturity. — The relations thus far described 

 are those that exist after the descendant clones have reached sexual or 

 reproductive maturity. But for a long period after conjugation, varying 

 in different clones from a few weeks to more than a year, the descendant 

 clones are in Paramecium bursaria sexually immature. During this period 

 the descendant clones do not mate at all, with any of the mating types. 

 In group 1, if they are mixed with mature individuals of any of the four 

 mating types A, B, C, D, there is no mating reaction, no formation of 

 pairs. In this period descendant clones show the characters of no mating 

 type. Later begins a period of partial maturity, in which a few members 

 of the clone pair with mature members of certain of the mating types. 

 During the early part of this period of partial maturity there are in 

 variety 1 only two mating types instead of four. Some of the descendant 

 clones form pairs with the types A or B, but not with C or D; others 

 with C or D, but not with A or B. The former may now be said to con- 

 stitute the type CD, the latter the type AB. Later these two young types 

 become further differentiated. Some of the clones that thus far do not 

 react with A or B acquire the power to react with A, but still do not 

 react with B; these now belong to the definitive mating type B. Others 

 acquire the power to react with B but not with A; these belong to the 

 definitive type A. A similar differentiation occurs among the clones of 

 the young type CD; some of these become type C, others type D. Thus the 

 types A and B are closely akin, being for a time one type AB; similarly, 

 types C and D at first constitute one type, CD. The interest of these 

 gradual changes in maturity and type during vegetative reproduction has 

 been emphasized in an earlier section (p. 714 above) . 



