428 A CENTURY OF SCIENCE 



and developing organs to a great variety of artificial con- 

 ditions. 



Artificial Parthenogenesis. Another question concerns 

 the nature of the process of fertilization and the agencies 

 which cause the fertilized egg to develop into an embryo. 

 In 1899 Jacques Loeb succeeded in causing development 

 in unfertilized sea-urchin eggs by subjecting them to con- 

 centrated sea water for a period and then returning them 

 to their normal environment. To this promising field of 

 experimental work came many of the foremost biologists 

 both in America and Europe. It was soon found that 

 the eggs of most groups of animals except the higher 

 vertebrates could be made to develop into more or less 

 perfect embryos and larval forms by treatment with a 

 great variety of chemical substances, by increased tem- 

 perature, by mechanical stimuli and by other means. 

 This artificial parthenogenesis, as it is called, has also 

 been successful in plants (Fucus), and recently Loeb has 

 reared several frogs to sexual maturity by merely 

 puncturing with a sharp needle the eggs from which they 

 were derived. Loeb, then, maintains that ' ' the egg is the 

 future embryo and animal; and that the spermatozoon, 

 aside from its activating effect, only transmits Mendelian 

 characters to the egg. ' ' 9 



Further experimental analyses of the nature of the fer- 

 tilization mechanism have recently been made by Mor- 

 gan, Conklin, F. R. Lillie, and others. 



Germinal Localisation. The question as to whether the 

 egg contains localized organ-forming substances has been 

 studied experimentally particularly by means of the cen- 

 trifuge. The results indicate that neither of the older 

 opposing theories of "performation" or "epigenesis" is 

 applicable to all eggs, but that in certain organisms the 

 eggs possess a well-marked differentiation while in 

 others each part of the egg is essentially, although prob- 

 ably not absolutely, equipotential. 



The Germplasm Cycle. Since "Weismann's postula- 

 tion of the independence of soma and germplasm in 1885 

 many attempts have been made to trace the path of the 

 hereditary substance from one generation to the next. 

 A recent book by Hegner 10 summarizes the success 

 attained in various groups of animals. 



