38 THE PHYSICAL BASIS OF INHERITANCE 



different from the ordinary cells of the body ? Let us, then, 

 concentrate our attention for a little on the nature and origin of 

 the germ-cells. 



It is inexpedient to lay on the shoulders of the student of 

 heredity the burden of problems which are not in any special 

 sense his business. It is no doubt interesting to ask how an 

 organisation, supposed to be very complex, may be imagined to 

 find physical basis in a microscopic germ-cell, but the same sort 

 of question may be raised in regard to a ganglion-cell. It is not 

 distinctively a problem of heredity. It is interesting to inquire 

 into the orderly and correlated succession of processes by which 

 the fertilised egg-cell gives rise to an embryo, but this is the 

 unsolved problem of physiological embryology. It raises 

 questions distinct from those of heredity and inheritance, and 

 apparently much less soluble. 



We shall return in the historical chapter to the various theories 

 of heredity which have been suggested ; in the meantime, we 

 require to refer to them only in outline. 



The Typical Ovum. — The germ-cell produced by the maternal 

 parent is usually a relatively large sphere of living matter (cyto- 

 plasm), and various not-living included substances, such as 

 nutritive yolk, pigment, oil-globules, and so forth. In the 

 cytoplasm there lies a central kernel surrounded by a delicate 

 membrane, the nucleus — a microcosm in itself. It contains 

 a network or coil or some arrangement of delicate (linin) threads, 

 carrying minute masses of a readily stainable material, the 

 chromatin. Under high magnification the chromatin is seen to 

 be built up of smaU corpuscles, sometimes like beads on a string, 

 the microsomes. In certain phases of activity the chromatin 

 forms a definite number of separate masses. They are then 

 called chromosomes or idants, and the same number is always 

 present in all the cells of the body of any particular species. 

 In the nuclear sap which fills the nucleus there is often a 

 rounded body or vesicle — the nucleolus ; or there may be 



