BEARERS OF THE HEREDITARY QUALITIES 59 



is typically as different as possible from that of 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 para- 

 mount importance in inheritance. 



3. Argument from fertilisation. In typical cases of fertilisa- 

 tion in animals, and in many plants as well, a spermatozoon 

 enters an ovum, sometimes a hundred thousand times larger 



\ 



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

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



than itself. As it enters it may leave behind it the locomotor 

 " tail," which has 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 



