DEVELOPMENT OF THE EXTERNAL FORM OF THE BODY. 



117 



cervical flexure is still prominent, as is also the ventral rotundity of the 

 body. The neck is now clearly differentiated. The external auditory 

 meatus and the surrounding rudiments of the concha are plainly indicated. 

 The limb buds are turned more nearly at right angles to the long axis of the 

 body. The leg and thigh show early differentiation; the fingers are beginning 

 to elongate and radial grooves on the foot indicate the boundaries between 

 the toes. The tail, which was a prominent feature in the earlier stages, is 

 proportionately small; in the human it is at most a rudimentary structure 

 represented by the coccyx, and while in the early embryo it is fairly large 

 it does not keep pace with the body during development. 



FIG. 94. Human embryo of 78 mm. 

 (3 months). Minot. 



FIG. 95. Human embryo of 4 months 

 Natural size. Kollmann. 



After the stage shown in Fig. 90 the cervical flexure continues to dimin- 

 ish and the head comes to lie more nearly in line with the long axis of the 

 body. The rotundity of the abdomen gradually becomes less as the heart 

 and liver approach the proportions of the adult. The tail as an external 

 structure disappears altogether, the buttocks increasing markedly. During 

 the second month the external genitalia develop and the sex of the embryo 

 can be distinguished. The general changes in form can be followed by com- 

 paring Figs. 91, 92, 93, 94 and 95. 



In the early stages of human development, say during the first month, 



