Vlll CONTENTS 



CHAPTER VIII 



Page 

 The Changes Between Twenty-four and Thirty-three Hours of 



Incubation 52 



The closure of the neural tube; the differentiation of the brain region; _, 

 the anterior neuropore; the sinus rhomboidalis; the fate of the primitive 

 streak; the formation of additional somites; the lengthening of the 

 fore-gut; the appearance of the heart and the omphalomesenteric 

 veins; organization in the area vasculosa. ^ 



CHAPTER IX 



The Structure of Chicks Between Totrty-three and Thirty-nine 



/ Hours of Incubation 59 



(The divisions of the brain and their neuromeric structure; the auditory 

 pits; the formation of extra-embryonic blood vessels ;^the formation of 

 the heart; the formation of intra-embryonic blood vessels. ; 



CHAPTER X 



The Changes Between Forty and Fifty Hours of Incubation 75 



^Flexion and torsion; the completion of the vitelline circulatory channels; 

 . the beginning of the circulation of blood. 



CHAPTER XI 



Extra-embryonic Membranes 80 



The folding off of the body of the embryo; the establishment of the 

 yolk-sac and the dehmitation of the embryonic gut; the amnion and the 

 serosa; the allantois. 



CHAPTER XII 



The Structure of Chicks from Fifty to Fifty-five Hours of In- 

 cubation 93 



I. External Features. 

 II. The Nervous System. 



Growth of the telencephalic region; the epiphysis; the in- 

 fundibulum and Rathke's pocket; the optic vesicles; the lens; 

 the posterior part of the brain and the cord region of the neural 

 tube; the neural crests. 



III. The Digestive Tract. 



The fore-gut; the stomodaeum; the pre-oral gut; the mid-gut; 

 the hind-gut. 



IV. The Visceral Clefts and Visceral Arches. 

 V. The Circulatory System. 



The heart; the aortic arches; the fusion of the dorsal aortae; the 

 cardinal and omphalomesenteric vessels. 

 VI. The Differentiation of the Somites. 

 VII. The Urinary System. 



