BEARERS OF THE HEREDITARY QUALITIES 59 



is typically as different as possible from that ot the very minute, 

 actively mobile, usually short-lived spermatozoon. The constancy 

 and frequent complexity of the reduction-processes which secure 

 the equivalence of chromosomes suggest that these bodies are of 

 paramount importance in inheritance. 



3. Argument from fertilisation. In typical cases of fertilisation in 

 animals, and in many plants as well, a spermatozoon enters an 

 ovum, sometimes a hundred thousand times larger than itself. As 

 it enters it may leave behind it the locomotor " tail," which has 



FIG. 14. The chromatin elements of the nuclei in coil (a), double star (6), 

 and almost divided stages (c). (After Pfitzner.) 



discharged its function,, thus further reducing its infinitely small 

 stock of cytoplasmic material. The " head " of the spermatozoon, 

 which is mostly nucleus, and the little " middle piece " which carries 

 the centrosome, are apparently the important parts, and it is the 

 ovum which furnishes the cytoplasmic basis of further operations. 

 The very gist of fertilisation, so far as we can see it, is the intimate 

 and orderly combination of the paternal and maternal chromosomes 

 to form one nucleus the segmentation-nucleus. Moreover, the 

 maternal and paternal contributions are, as we have noted, distributed 



