DEVELOPMENT OF THE EXTERNAL FORM OF THE BODY. 



Ill 



flexure, the oral fossa, the protrusion in the cervical region caused by the 

 heart, the belly stalk, and the constriction between body and yolk sac are all 



Fore-brain 



Mid-brain 



Hind-brain 



Omphalornesenteric 



- Yolk sac 



**. Amnion 



Belly stalk 



FIG. 84. Dorso-lateral view of human embryo with fourteen pairs of mesodermal 

 somites (2.5 mm.). Kollmann. 



clearly indicated. It is worthy of note that the heart appears in the cervical 

 region; during later development it recedes into thorax. 



Second branchial arch 

 Third branchial groove 

 Heart 



FIG. 85. Human embryo of 2.6 mm. His, from Keibel and Mall. 



One of the early human embryos described by His is shown in Fig. 85. 

 The veil-like structure around the embryo is the amnion. This embryo 

 measures 2.6 mm. and was estimated to be 18-21 days old (the estimate in 



