THE RISE OF EMBRYOLOGY 73 



published in 1864, gives an interesting account of 

 his mental development. 



Between von Baer and Balfour (1851-1882) under 

 whom embryology took on modern aspects, the 

 observations on the development of animals were 

 greatly multiplied. Activity was increased after the 

 announcement of the cell- theory (1839) and em- 

 bryologists began to see more clearly the significance 

 of the germinal elements and the germ-layers. 



It was determined that the egg and the sperm are 

 single cells, the final step in reference to all eggs 

 being taken by Gegenbaur in 1865. The three germ- 

 layers common to all animals above the Ccelenterates 

 (Hydra, Hydroids, Jelly-fish, etc.) were shown to be 

 essentially alike as to origin and to give rise to the 

 same kind of tissues in the different animals. In the 

 light of embryology, all animals were seen to be 

 related through ancestral lines and to be united on the 

 broad plane of similarity of origin and development. 

 Von Baer had already indicated this, but it was in the 

 period between him and Balfour, that this great 

 truth became better illustrated and took hold on the 

 minds of embryologists and influenced their in- 

 vestigations. 



Balfour (Fig. 17) now comes upon the scene and 



