THE PHYSICAL BASIS OF HEREDITY. 169 



in order that they may be able to come together in the 

 water in which they are usually set free. The second 

 requires that there be furnished a sufficient amount of 

 nutritive material for the nourishment of the embryo 

 until it arrives at a stage of growth in which it can shift 

 for itself. These two necessities have been met by a 

 physiological division of labour between the two con- 

 jugating cells. The one, the sperm cell, has become 

 reduced in size with a corresponding gain in motility, 

 the other, the egg cell, has had food yolk stored up in 

 it, and its consequent increased size prevents any more 

 than a very slight degree of independent movement, if 

 any. Different stages of these modifications may be 

 met with among unicellular forms, as illustrated above 

 in Faiidortna, Eudori/uiy and Voh>ox, to which might be 

 added many others. In Pandorina the conjugating cells 

 are of nearly equal size, in Eudorina an intermediate 

 condition is reached, while in Volvox the tgg and sperm 

 cells are sharply differentiated in size and motility. 

 Again, in the first two and their allies all oi the cells are 

 at first vegetative and afterward reproductive, while in 

 Volvox the definite separation into vegetative or 

 somatic, and reproductive or germinal cells makes its 

 appearance. 



We arrive then at the conclusion, from the considera- 

 tion of these and many other lines of evidence, that the 

 germ cells were primitively exactly alike, 

 Fundamental ^^^ ^y^^^ ^^^ differences between them 

 identity of the 



germ cells. have arisen in the process of differentia- 



tion along two separate lines. Further- 

 more, it is clear that the dififerences between the two 

 sexes, which become strongly characterized in the higher 

 vertebrates, are all of a purely secondary nature. 



In their early development the germ cells are indis- 

 tinguishable from each other, and both pass through 



