OUR NATURAL INHERITANCE 57 



cell (the spermatozoon) which unite at the beginning of 

 each new life. 



(3) The inheritance, contained implicitly in the 

 fertilised egg-cell, requires an appropriate nurture if 

 it is to develop aright. Development is the making 

 visible or actual of what has lain in the germ-cell in 

 an invisible or potential state. 



(4) In mammals, and in some other cases, e.g. 

 flowering plants, the developing embryo may be 

 influenced very early by its immediate surroundings 

 within the mother, but that influence is part of nurture, 

 not part of the hereditary nature. 



(5) Apart from a few exceptional cases, e.g, virgin 

 birth (parthenogenesis), which is restricted in natural 

 conditions to backboneless animals, every inheritance 

 is dual, partly paternal and partly maternal. The 

 mother certainly contributes in the cytoplasm or the 

 extra-nuclear substance of the egg-cell the greater 

 part of the initial building material, the sperm-cell 

 being very much more minute. It is highly probable 

 that many old-established generic characters have their 

 vehicle in the cytoplasm of the ovum. As regards 

 those hereditary items which are carried in the nuclei 

 of the sex-cells, it is interesting to notice that the 

 nuclear-bodies (chromosomes) are usually equal or 

 nearly equal in number in sperm and ovum. Each 

 kind of organism has a definite number of chromosomes 

 which is usually the same in all the cells, except the 

 unripe ova and sperms, which have double the normal 

 number. There is nothing in the number itself, which 



