388 THE WONDER OF LIFE 



way of development, the spermatozoon may do yet 

 another thing. In some insects and other types, half of 

 the spermatozoa have the same number of chromosomes as 



the ripe egg f ^ ], while half of them have one fewerf -— 1 j, 



and there seems to be good evidence that when two equal 



- + - ) the result is a female, 



while an ovum fertiUzed by a spermatozoon with -—1 



chromosomes develops into a male. 



We see, then, how much is involved when a spermatozoon 

 fertihzes an ovum. There is a minghng of the paternal 

 and maternal inheritances ; there is a restoration of the 

 normal number of chromosomes ; there is the introduction 

 of the minute centrosome which plays an important role in 

 cleavage ; there is an activation of the egg and a stimulus 

 to embryo-forming ; and there is a rapid change effected 

 in the periphery of the ovum, so that it becomes non- 

 receptive to other spermatozoa. 



A ghmpse into the subtleties that lie beyond may per- 

 haps be given by taking a particular item of fact. Giinther 

 Hertwig finds that the eggs of the Edible Frog {Rana 

 esculenta) and the Common Toad (Bufo vulgaris) may be 

 fertihzed by sperms of the Brown Frog {Rana fusca). They 

 segment normally, but they die before they reach the 

 gastrula stage of development. But if the spermatozoa 

 of Rana fusca be first exposed to intense Radium rays, 

 and then used for f ertihzation, the eggs develop into larvae 

 which survive for several weeks. The explanation sug- 

 gested of this curious paradox may be wrong, but it is 

 illustrative. It is this, that the spermatozoa of R. fusca 



