4 JACQUES LOEB 



segment until after eight hours.'' I have repeated these experi- 

 ments in a number of fish hybrids and confirmed Moenkhaus' 

 results. The same facts are observed in the rate of development 

 of hybrid embryos of echinoderms. What is the meaning of this 

 fact? I believe that it finds its explanation through artificial 

 parthenogenesis. These latter experiments have shown that the 

 spermatozoon does not cause the development by carrying an 

 enzyme or katalyzer into the egg, which the latter needs in order 

 to develop, but causes the development by altering the surface 

 layer of the egg. If the development of the egg were caused by 

 an enzyme carried into the egg by a spermatozoon, the rate of 

 cleavage of slowly developing eggs should be accelerated by a 

 spermatozoon of a species developing at a faster rate. The egg 

 however behaves exactly as we should expect from the fact that the 

 spermatozoon removes only certain obstacles for the development 

 of the egg but does not cause its development by carrying an acti- 

 vating enzyme. In order to cause the parthenogenetic develop- 

 ment of the sea-urchin egg two different agencies are required and 

 I have been able to show that the spermatozoon also causes the 

 development of the sea-urchin egg by two agencies which act 

 analogously to the agencies employed in my method of artificial 

 parthenogenesis.'^ F. Lillie has found the same in Nereis.^ It was 

 therefore to be expected that the rate of development of embryos 

 was determined by the egg. This view meets with a difficulty 

 in the fact that, with a few exceptions, the later development of 

 hybrids is as a rule retarded. But this difficulty is only an appar- 

 ent one since the retardation is due to an entirely secondary con- 

 dition : namely that most hybrids are sickly and not able to hatch 

 or reach an adult state. 



This is most strikingly the case in the heterogeneous hybrid, 

 e.g., in the cross between the sea-urchin and the starfish. As I 

 pointed out long ago the larvae die mostly in the gastrula stage, 



* The acceleration of segmentation which Newman observed in the egg of Fun- 

 dulus majalis fertilized by the sperm of Fundulus heteroclitus is too small to influ- 

 ence our conclusions. 



* Loeb, Die chemische Entwicklungserregung des thierisches Eies. Berlin, 

 1909, and Das Wesen der formativen Reizung, Berlin, 1909. 



' F. R. Lillie, Jour, of Morph., vol. 22, 1911. 



