6 CONTENTS 



The last third of the ninth day. 



Stage 26. Cephalic flexure. Beginning of amnion formation. 

 Fusion of the hearts in the ventricular region. Origin of the 

 postcardinal vein. Miscellaneous details. Comparative notes. 

 General comments on comparative data. 



X. The tenth day. Stages 27 to 29 91 



The first third of the tenth day. 



Stage 27. External distinctive features. The cervical flexure. 

 The lung buds. The tail fold of the amnion. Changes in the 

 vascular system. Miscellaneous details. 

 The middle of the tenth day. 



Stage 28. External distinctive features. The primary lumbar 

 flexure. The mesonephric tubules. The liver divertieulum. 

 The paracardinal plexus. The veins of the limb buds. The 

 allantois. Miscellaneous details. 

 The last third of the tenth day. 



Stage 29. External distinctive features. The extra-embryonic 

 membranes. The pharyngeal bursa. The dorsal pancreatic 

 diverticulum. The thyroid anlage. Miscellaneous details. 

 Comparative notes. 



XI. The eleventh day. Stages 30 and 31 125 



The first half of the eleventh day 



Stage 30. External distinctive features. The secondary lumbar 

 flexure. Bronchiolar buds. The pulmonary arteries. The 

 liver cords. The septum transversum and the pleuroperi- 

 cardial membrane. Advances in the nervous system. Miscel- 

 laneous details. 

 The second half of the eleventh day. 



Stage 31. External distinctive features. Changes in the chorion. 

 Changes in the venous system. The earliest lymphatic anlagen. 

 Changes in the arterial system. Changes in the pancreas. 

 Pleuriperitoneal membranes. The ureteric bud. The primitive 

 choanae. The motor nerves of the eye. Miscellaneous details. 



XII. The twelfth day. Stages 32 and 33 144 



The first half of the twelfth day. 



Stage 32. External distinctive features. The veins of the liver 

 and mesonephros. The thoracic duct and the jugular lymph 

 sacs. The adrenal cortex. The foramen ovale. The subin- 

 testinal vein. Changes in the branchial pouches. Changes in 

 the digestive tract. Miscellaneous details. 

 The second half of the twelfth day. 



Stage 33. External distinctive features. The veins of the liver 

 and the postcava. The septum primum. The jugular lymph, 

 sacs. The thoracic supracardinal (azygos) veins. The para- 

 vertebral lymphatics and the thoracic duct. Migration of the 

 gill pouch derivatives. The allantois at its height. Miscel- 

 laneous details. 



