CONTENTS i x 



CHAPTER XI 



PAGE 



The Effect of Injuring One of the First Two Blastomeres 106 

 Roux's Experiment of " Killing " One of the First Two Blasto- 

 meres. 

 Further Experiments by Others (Hertwig, Endres and Walter, 

 Schultze, Wetzel, Morgan). 



CHAPTER XII 



Interpretations of the Experiments ; and Conclusions . . 123 

 Roux's Mosaic Theory of Development. 

 Theory of Driesch and of Hertwig of the Equivalency of the Early 



Blastomeres. 

 Roux's Subsidiary Hypothesis. 

 Experiments on Other Forms. 

 General Conclusions. * 



CHAPTER XIII 



Organs from the Endoderm 137 



The Closure of the Blastopore, and the Formation of the Neuren- 



teric Canal. 

 The Digestive Tract and the Gill-slits. 



CHAPTER XIV 



Organs from the Mesoderm 146 



The Mesodermic Somites. 

 The Heart and Blood-vessels. 

 The Pronephros. 



CHAPTER XV 



Organs from the Ectoderm 159 



The Central Nervous System. 



The Eyes. 



The Ears. 



The Nerves. 



The Appearance of Cilia on the Surface of the Embryo. 



