REPRODUCTION AND DEVELOPMENT 



61 



regions in the trunk. Dorsally, the auricle can be distinguished from 

 the ventral and thicker-walled ventricle. The position of the liver is 

 indicated by the swelling between the heart and the anterior limb bud. 



omn/'ot/'c fo/d 



egr^5/>e// 

 egg-membrane 



- omniotic fo/of 



- ~ en7/:?ryo 



'amnion 



O //an to/5 



-- c/7or/on 



^^c/o/Zc-sac 



a//antoic cai'/ty 



a//antoi5 

 amn/oC/c cai^/tg 

 -amn/or? 

 a//anto/c 3Ca/A 

 i/o/^-3ta/k 

 chorion 



Fig. 33. — The embryonic membranes of the chick; diagrams of longitudinal 

 sections of the embryo. A, by the end of the second day of incubation both 

 the anterior and posterior amniotic folds, the fusion of which gives rise to 

 the anmion and chorion, can be seen. B, by the end of the third day the 

 amniotic folds have united and the allantois has arisen as an outpocketing 

 from the gut and grown into the cavity between the amnion and chorion. 

 Note that the yolk sac, or cellular membrane surrounding the yolk, is con- 

 tinuous with the lining of the gut. C, from the fifth day on, the allantois 

 grows rapidly and fills all available space; its outer layer becomes closely 

 applied to the chorion, while the inner layer hes next to the white of the egg. 



(Redrawn with modifications from T. J. Parker and W. A. Haswell, "Textbook of 

 Zoologj'," copyright, 1921, by Macmillan and Co., Ltd., printed by permission.) 



Dorsal to the limb bud the mesodermal somites are conspicuous for 

 the entire length of the trunk. Between the anterior and posterior 

 limb bud the mesonephros forms a prominent mass. Posterior to the 



