EEPBODUCTION AND HEREDITY '295 



becomes aggregated around the others. The result of the first 

 division is therefore again two cells, one large and one small, 

 but each with the normal number of four chromosomes. The 

 second division, which follows without an intervening rest-period, 

 effects again a reduction in chromatin, removing two of the 

 chromosomes, each of which is surrounded by a minute quantity 

 of protoplasm from the ovum. In the meantime, the first small 

 cell has once more divided, and with that there have again been 

 formed from the oocyte of the first order four cells, the mature 

 ovum, and the three ' directive-cells ' which soon perish without 

 having performed any particular role. 



It may be asked, Why does Nature act in this case so un- 

 justly? Why do not four normal ova develop from the oocyte? 

 But the reason seems plain : we see here once more manifested 

 the great adaptive faculty of organic genesis. As the egg-cell 

 is to supply the formative matter for the growing embyro, is 

 it not above everything necessary that it is liberally provided 

 with food-material so that it shall be able to fulfil its task? 

 Thus it seems more advantageous that fewer ova are created and 

 that one cell enriches itself at the expense of the other three 

 sister-cells, than that more but weaker germs are produced. 



When both kinds of sex-cells have been prepared as described, 

 only then can the process take place at which all these prepara- 

 tions aim, fertilization. It must have been a wonderful im- 

 pression which Oscar Hertwig received in 1875 when he, first of 

 all men, was able to observe with a microscope these mysterious 

 processes in the ova and spermatozoa of sea-urchins, and thus 

 open a path to a region which the human mind had during 

 thousands of years in vain endeavoured to invade. Even to-day, 

 when anyone with a good magnifying glass can easily observe 

 the spectacle, it never fails to fascinate us. We feel that we 

 stand here on the threshold of one of the profoundest riddles 

 of life. 



The process of fertilization is exceeding!)*- simple. As soon 

 as we mix in a drop of sea-water a few mature ova of the sea- 

 urchin with spermatozoa, we see that immediately all the sperma- 

 tozoa rush towards the ova, each one endeavouring to reach 

 an ovum before the others. When a spermatozoon is close to an 

 egg-cell the egg-plasm raises a minute prominence into which 



