4 o THE PHYSICAL BASIS OF INHERITANCE 



a current if there is one. It is obviously a specialised adaptation 

 which helps the spermatozoon to find the ovum, and it may be 

 absent in cases where no journey or search is required. The 

 so-called head of the spermatozoon contains the stainable 

 material or chromatin, and in many cases it has been shown that 

 the ripe spermatozoon has the same number of chromosomes as 

 the ripe ovum. At the junction of the " head " and the " tail " 

 there is a short " middle piece " or " neck," in which there is 

 often seen a minute " centrosome." 



There is in animals in most cases a great superficial contrast 

 between the two kinds of "germ-cells when fully mature. The 

 typical ovum is relatively large, often laden with yolk, usually 

 passive, and surrounded by some sort of membrane. The 

 typical spermatozoon is relatively very minute, with no 

 reserve material, and adapted to active locomotion. It is 

 significant, however, that both contain the same number of 

 chromosomes. 



Old Attempts to interpret the Uniqueness of the Germ- 

 cells. — In the preformationist theories, which held sway in the 

 seventeenth and eighteenth centuries — theories which asserted 

 the pre-existence of the organism and all its parts, in miniature, 

 within the germ — there was a kernel of truth well concealed 

 within a thick husk of error. For we may still say, as the 

 preformationists did, that the future organism is implicit in the 

 germ, and that the germ contains not only the rudiment of the 

 adult organism, but the potentiality of successive generations 

 as well. But what baffled the earlier investigators was the 

 question, How the germ-cell comes to have this ready-made 

 organisation, this marvellous potentiality. Discovering no 

 natural way of accounting for this, the majority fell back upon 

 a hypothesis of hyperphysical agencies — that is to say, they 

 abandoned the scientific method, and drew cheques upon 

 that bank where credit is unlimited as long as credulity 

 endures. 



