FACTS OF INHERITANCE 137 



seem to be equally valid, and though the generalisa- 

 tions reached along the different lines do not at 

 present cohere in a harmonious body of doctrine, 

 there is no reason to doubt that this will gradually 

 develop. Let us illustrate some of the results 

 attained along the three lines. 



MICROSCOPICAL STUDY OF THE GERM-CELLS. 

 Most plants and animals are built up of cells 

 and start in life as fertilised egg-cells, and it 

 was in a fertilised egg-cell that our own natural 

 inheritance consisted. A few exceptions may be 

 made e.g. for bananas, which have no longer 

 any seeds ; for potatoes, which are multiplied by 

 cutting; for the drone-bees and summer green- 

 flies, which have mothers but no fathers; and 

 for the simple, single-celled organisms which are 

 themselves comparable to eggs and sperms ; but 

 the exceptions are trivial compared with the 

 vast majority of living creatures of which it is 

 certain that each individual life begins as a ferti- 

 lised egg-cell the result of the intimate and 

 orderly union of a spermatozoon and an ovum. 



We get a very misleading idea of the ovum, or 

 egg-cell, when we think, as we always do at first, 

 of birds' eggs. For in these familiar objects the 

 true egg-cell has been dilated by an enormous 

 quantity of nutritive yolk, on the top of which 

 a minute drop of nucleated living matter lies 

 like an inverted watch-glass. Most ova are very 

 minute cells, often invisible to the naked eye. 

 The spermatozoon, or male element, which fertilises 

 the egg, is smaller still ; it is often only 10 Q 000 th 

 of the ovum's size. In a way that we cannot 

 picture each of the germ-cells (or gametes) carries a 

 complete set of hereditary characters. All theory 



