THE PHENOMENA OF MATURATION IN PROTOZOA 169 



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pole of the nuclei. The first division of the fertilization nucleus take: 

 place before the chromatin of the two nuclei is completely united. 

 The other two divisions of the fertilized nucleus follow in quick suc- 

 cession and the processes of reorganization bring the phenomena of 

 conjugation to an end (see Fig. 6S, p. 157). 



The mere statement of the consecutive acts in maturation and 

 fertilization gives no clue to the significance of the processes whereby 

 the cell is reendowed with a potential of vitality which will again carry 

 it through the periods of a life cycle. A¥e see that fully three-quarters 

 of the chromatin of the resting nucleus is eliminated to disintegrate in 

 the protoplasm of the cell, while still more is lost in the material of the 

 connecting strands of the daughter nuclei; we see that there is a union 

 of this reduced ("purified") chromatin when the pronuclei come 

 together, and we see that the new macronucleus of the early genera- 

 tions of cells is derived from part of the fertilization nucleus. 



It thus appears that the nuclear parts of the fertilized cell of Para- 

 mecium are distinctly new creations, for they consist of the union of 

 chromatin material from two distinct organisms ; the one, the macro- 

 nucleus, has from this period the essentials of vegetative activity, 

 while the micronucleus apparently enters into a "resting period," 

 dividing, and possibly controlling, cell division, until the next period of 

 sexual activity. Not only is the nuclear apparatus new in the fertilized 

 cell, but the cytoplasm is also new, for it receives and changes over into 

 its own substance not only the remains of the old macronucleus, but 

 more than three-fourths of the entire quantity of chromatin possessed 

 by the maturation nuclei. There is no reason to doubt that this addi- 

 tion makes over the protoplasm of the cell in a manner analogous to 

 the reorganization of the nuclei, and presumably provides a physical 

 basis for the reinvigoration of the activities pecufiar to the cytoplasm. 

 Except for the absence of a cellular corpse, therefore, there is no sup- 

 port for Weismann's contention that the old individual still persists; 

 it is a new individual, nucleus and cytoplasm, no less surely than a 

 fertilized ovum, or its progeny of cells, is a new individual. 



The secret of development lies in this fertilization act, and if we 

 could work it out in paramecium and its allies, we would have a basis, 

 at least, for its discovery in the higher animals. The protozoon offers 

 a much more suitable organism for the study of this problem than any 

 many-celled animal, for the conditions under which successful con- 

 jugation is brought about may be experimentally studied and con- 

 trolled. Not much has been done, as yet, in this direction, but numer- 

 ous observers are at work on the problem, and it thus presents one of 

 the most fascinating aspects of protozoology. 



It is mainly in connection with such complicated phenomena of 

 chromosome formation and reduction that theories of inheritance and 

 of fertilization have been formulated. 



