502 THE HUMAN EMBRYO. 



degree to which the head is lifted iip, by straightening of the 

 cervical flexure, than on any real increase in the dimensions. 



The position of the embryo within the uterus, about the 

 fortieth day, is shown in Fig. 210. The embryo is connected 

 with the placenta by a thick allantoic stalk. The yolk-stalk is 

 long and thin : its proximal part is bound up with the allantoic 

 stalk in a sheath formed round both by the inner or true am- 

 nion ; while its distal portion, ending in the small yolk-sac, lies 

 between the amnion and the chorion. The amnion is a trans- 

 parent sac some distance from the embryo. 



The embryo itself is rapidly assuming more definite shape, 

 and by the end of the week is distinctly human in appearance. 

 Owing to the thickening of the muscles and of the subcutaneous 

 connective tissue, and the formation of skeletal elements, the 

 shape of the embryo as a whole, and especially of the head, is 

 much less dependent on the internal organs than in the earlier 

 stages. 



The head is still of great size. The face has made consider- 

 able progress, and the features are now well established. The 

 nose is larger than before, but is still very broad and flat. The 

 eyelids are commencing to form, as folds of skin, above and below 

 the eyes. The lips appear as folds at the margins of the jaws, 

 but only reach a small development during the sixth week : the 

 red ridge of each lip arises independently, and not until a much 

 later period ; about the middle of the third month. 



Up to the end of the fifth week there is a distinct notch in 

 the median plane where the two mandibular arches meet : 

 during the sixth week this notch is gradually filled up, and the 

 chin formed as a median projection. 



The external ear makes considerable progress during the 

 week (Figs. 209, 211), and by its close the relations and propor- 

 tions of the several parts are readily comparable with those of 

 the adult. 



Apart from the external auditory meatus and the external 

 ear, the visceral clefts and arches are no longer recognisable. 

 The sinus prsecervicalis has closed, and the neck is becoming 

 established as a constricted region between the head and 

 trunk. 



The limbs have increased considerably in size, the upper 

 arm and thigh in particular being much longer than before. 



