vi CONTENTS 



PAGE 



CHAPTER IV. FROM LAYING TO THE FORMATION OF 



THE FIRST SOMITE 69 



I. Structure of the Unincubated Blastoderm .... 69 



II. The Primitive Streak 69 



Total Views 69 



Sections 74 



The Head-process 80 



Interpretation of the Primitive Streak ...... 83 



III. The Mesoderm of the Opaque Area 86 



IV. The Germ-wall 90 



CHAPTER V. HEAD-FOLD TO TWELVE SOMITES 



(From about the twenty-first to the thirty-third hour of incu- 

 bation) 91 



I. Origin of the Head-fold 91 



II. Formation of the Fore-gut 93 



III. Origin of the Neural Tube 95 



The Medullary Plate 95 



The Neural Groove and Folds 97 



Primary Divisions of the Neural Tube 105 



Origin of the Primary Divisions of the Embryonic Brain 108 



IV. The Mesoblast 109 



Primary Structure of the Somites 114 



The Nephrotome, or Intermediate Cell-mass (Middle 



Plate) 114 



The Lateral Plate 115 



Development of the Body-cavity or Coelome 115 



Mesoblast of the Head 116 



Vascular System 117 



Origin of the Heart 119 



The Embryonic Blood-vessels 121 



V. Description of an Embryo with 10 Somites .... 122 



The Nervous System 124 



Alimentary Canal 126 



Vascular System 126 



General 127 



Zones of the Blastoderm 127 



CHAPTER VI. FROM TWELVE TO THIRTY-SIX SOMITES. 



THIRTY-FOUR TO SEVENTY-TWO HOURS . 130 

 I. Development of the External Form, and Turning of 



the Embryo 130 



Separation of the Embryo from the Blastoderm . . . 130 



