THE METHOD OF EUGENICS 13 



that the number of these determiners should double in 

 every succeeding generation. There must be some special 

 mechanism to prevent this result. An appropriate mechan- 

 ism is, indeed, ready and had been seen and studied long 

 before its significance was understood; this is the elimina- 



FiG. 3. — Three stages in the fertilization of the egg of a marine ringed 

 worm (Tfialassema). As seen in thin dyed sections. A. At the top of the egg 

 there is occurring a division of the chromosomes that constitutes the ripening 

 or "maturation'' of the egg, illustrated in greater detail in Fig. 4. At the bot- 

 tom a sperm cell (c^) has entered the egg and is penetrating through it toward 

 its center. B. The nucleus of the egg is now returning toward the center to 

 meet that of the sperm. C. The egg and sperm nuclei are now in contact; 

 henceforth they work in unison; fertilization is completed. After Griffin 

 from E. B. Wilson: "The Cell in Development and Inheritance." 



tion from both the immature egg and the immature sperm 

 of half of the chromatic material (Fig. 4). Thus if the im- 

 mature sex-cell contains four chromatic bodies (chrom- 

 osomes) each mature sex-cell will contain only two chromo- 

 somes. Moreover, each of the chromosomes in the im- 

 mature sex-cell is double; one half having originated long 

 before in its maternal germ plasm and the other half in its 

 paternal germ plasm. The mechanism for maturation is 



process of splitting takes place. F. The processes just preceding chromosome 

 division are now completed; the activity of the centers is at its height; the 

 chromosomes now constitute an "equatorial plate," e. p. G. The chromosomes 

 at the equatorial plate are now beginning to move apart. H. The separation 

 of the chromosomes is continuing and in / is completed; meanwhile the ac- 

 tivity at the centers has declined and division of the body of the cell is begin- 

 ning. J. Division of the cell completed; the nuclei and centrosomes at the 

 condition with which we started at A. From E. B. Wilson: "The Cell in 

 Development and Inheritance." 



