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THE AMERICAN NATURALIST [Vol. XL VII 



which have been conducted on guinea-pigs for the last 

 few years. 



The spermatozoon is more difficult to treat experi- 

 mentally than is the ovum, on account of the fact that the 

 treatment so often renders it inactive or cripples it in 

 such a way that it is unable to penetrate and to fertilize 

 an egg. The spermatozoon, although apparently deli- 

 cate, is more or less resistant, so that a mild treatment 

 gives no effect. The difference between the upper non- 

 effective dose of treatment and the fatal or "paralyzing" 

 dose is slight, yet it is the precise treatment between 

 these two points which yields results. 



Little has been done in treating chemically the sperma- 

 tozoa of invertebrates, though some of the hybridization 

 experiments furnish indirect evidence as to what might 

 occur. Herbst's experiment of starting the development 

 of an egg by parthenogenesis and then fertilizing one 

 blastomere with a foreign spermatozoon offers splendid 

 opportunity for investigating the influence of strange 

 substances introduced by the sperm. The spermatozoa 

 of the sea urchin have been subjected to the action of 

 radium emanations by Giinther Hertwig, who found that 

 after intensive treatment for several hours such sperma- 

 tozoa subsequently disturb the processes of division m 

 the eggs which they have fertilized. Paula Hertwig found 

 that fertilized ascaris eggs treated several hours with 

 radium preparations gave pathological division figures; 

 the chromatin bodies showed a tendency to break down 

 and disintegrate ; division was slow and the karyokmetic 

 figures were finally completely deranged. 



Investigations on vertebrates have been more exten- 

 sive. The most beautiful results have been obtained on 

 fish and amphibians through the radium experiments of 

 Oscar Hertwig, his son Giinther and his daughter Paula. 

 The outcome of these experiments is so striking and is of 

 such serious importance that I shall dwell somewhat upon 

 their significance. About three years ago 0. Hertwig 

 published the results of his first radium experiments m 



