CHAPTER III. 



MODERN ONTOGENY. 



Karl Ernst Baer. 



Karl Ernst Baer, the Principal Disciple of Wolff. — The "Wiirzbnrg School of 

 Embryologists : DoUinger, Pander, Baer. — Pander's Theory of Germ- 

 layers. — Its Full Development by Baer. — The Disc-shaped first parts 

 into two Germ-layers, each of which again divides into Two Strata. 

 The Skin or Flesh-stratum arises from the Outer or Animal Germ-layer. 

 The Vascular or Mucous Stratum arises from the Inner or Vegetative 

 Germ-layer. The Significance of the Germ-layers. — The Modification 

 of the Layers into Tubes. — Baer's Discovery of the Human Egg, the 

 Germ-vesicle, and Chorda Dorsalis. — The Four Types of Evolution in 

 the Four Main Groups of the Animal Kingdom. — Baer's Law of the Type 

 of Evolution and the Degree of Perfection. — Explanation of this Law by 

 the Theory of Selection. — Baer's Successors : Kathke, Johannes Miiller, 

 Bischoff, Kolliker. — The Cell Theory : Schleiden, Schwann. — Its Appli- 

 cation to Ontogeny : Eobert Remak. — Retrogressions in Ontogeny : 

 Reichert and His. — E.vtension of the Domain of Ontogeny : Darwin. 



" The History of Evolution is the real source of light in the investigation 

 of organic bodies. It is applicable at every step, and all our ideas of the 

 correlation of organic bodies will be swayed by our knowledge of the 

 history of evolution. To cairy the proof of it into all branches of research 

 would be an almost endless task." — Karl Ernst Baer (1828). 



If we wish to separate our historic survey of the course 

 of the development of the Science of Human Ontogeny 

 into parts, it is most convenient to make three. The first 

 of these occupied the last chapter, and includes the whole 

 preparatory period of embryological researches ; it extends 



