DEVELOPMENT OF THE EXTERNAL FORM OF THE BODY. 



149 



After the stage shown in Fig. 126 the cervical flexure continues to dimin- 

 ish, so that the head comes to lie nearly in a direct line with the body (Fig. 127). 

 The rotundity of the abdomen diminishes owing to the fact that the heart and 

 liver grow more- slowly relatively to the body as a whole. The tail, which was 

 still a prominent feature in Fig. 125, continues to become less prominent in the 

 succeeding stages (Figs. 127, 128, 129, 130). This is not due so much to an 

 actual atrophy of the tail as to an increase in the size of the buttocks. In the 

 adult the only remnant of the tail is the coccyx. 



Fig. 129 



Fig. 130. 



Fig. 128. — Human embryo of 17.5 mm. (47-51 days). His. 

 Fig. 129. — Human embryo of 18.5 mm. (52-54 days). His. 

 Fig. 130. — Human embryo of 23 mm. (2 months). His. 



During the second month of development the external genitalia become very 

 prominent and the sexes can be easily differentiated. 



By the end of the second month the embryo has acquired a form which 

 resembles in a general way the form of the adult (Fig. 130). From this time on 

 it is customary to speak of the growing organism as afcetus. 



Branchial Arches — Face — Neck. 



At a very early stage (embryos of 2-4 mm.) certain peculiar structures 

 appear in that part of the embryo which is destined to become the face and neck 

 regions. They are at first noticeable as slit-like depressions nearly at right 

 angles to the long axis of the body. In an embryo 2.15 mm. long two of these 

 depressions are visible (Fig. 121). Shortly after this a third and then a fourth 



