Coe — A Century of Zoology in America. 389 



to the groups more difficult of investigation and into the 

 earliest stages of fertilization and implantation in the 

 mammals. Artificial cultural methods have yielded 

 important results. Louis and Alexander Agassiz, Mark, 

 Minot, Brooks, Whitman, Conklin and E. B. Wilson have 

 taken prominent parts in this work. 



In the early nineties embryological studies were 

 directed to the arrangement of cells in the dividing egg, 

 and there was much discussion of "cell lineage" in 

 development. Valuable as were these studies they threw 

 comparatively little light on the general problems of 

 evolution. 



Experimental Embryology. — A more fertile field, 

 developed at the same period and a little later, was found 

 in experimental embryology. The discoveries made by 

 Driesch and others in shaking apart the cells of the divid- 

 ing egg or by destroying one or more of these cells gave 

 a new insight into the potency of cells for compensatory 

 and regenerative processes. These studies attracted 

 many able investigators, who made still further advance 

 by subjecting the germ cells, developing eggs, embryos, 

 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, 



