14 HEREDITY 



is produced. The edges of this cup gi-adually grow 

 together until only a small opening remains, which is 

 the rudimentary mouth of the new animal. Very soon 

 the backbone begins to be sketched in, the head is 

 moulded off, and the hmbs commence to gro-\^' out. 

 The whole process is so remarkable that it looks as 

 though an invisible hand were at work, drawhig out 

 here, pinching there, and gradually moulding a complete 

 little copy of the parent form. 



It must indeed strike one as marvellous that all these 

 possibihties of gro\\-tli and development are hidden 

 away in a speck of hving jelly, several hundred times 

 less in volume than a pin-head. 



In plants, the bodies corresponding to the sperm and 

 egg-cells are contained in the pollen-grains and ovules 

 respectively. The structure of the reproductive bodies 

 differs somewhat from what we have described, and 

 the mechanism of fertilisation and the process of de- 

 velopment are not the same as in animals. Yet the 

 assential points of sexual reproduction are the same in 

 the two classes of hving things. 



According to Darwin's theory of heredity. Pangenesis, 

 each part of the body gave off minute buds or pangenes, 

 wliich passed by means of the blood to the reproductive 

 organs. Each egg-cell or sperm came to contain a set 

 of these buds, representing all parts of the body — skin 

 and hair, muscles, bones, heart, kidneys, and the rest. 

 In development, all the elements Mhich had been 

 packed away were simply unfolded and the child there- 

 fore came to resemble its parents. 



How many such buds would be necessary to give a 

 fair sample, as it were, of such a complex organism 

 as one of the liigher animals, it is impossible to say. 

 At any rate, the number would have to be enormous. 

 But many male animals and plants produce thousands 

 of millions of sperm cells in the course of their lives, 

 so that, if Pangenesis were to afford us an explanation 

 of heredity, we should have to suppose the giving off 

 of enormous numbers of complete sets of pangenes. 



This sketch of Darwn's theory v.i\\ be sufficient to 



