THE EARLY HUMAN EMBRYO. 



57 



the vitelline sac accomplished by the ventral approximation of the splanchnopleura, 

 and for a time maintains a wide communication with the remains of the yolk-cavity. 

 The early embryo, lying flatly expanded upon the blastodermic vesicle, becomes 

 differentiated in form by the appearance of head- and tail-grooves, in consequence of 



bl 



which constriction the cephalic and the caudal poles of the body become defined and 

 partially separated from the embryonal area ; the middle segment, however, em- 

 bracing the widely open gut-tract, for a time remains closely blended with the vitel- 

 line sac, of which, at first, the embryo appears as an appendage (Fig. 68, i and 2). 



