146 



THE HUMAN EMBRYO. 



specimen of this stage we may take the embryo designated by His as Br. i , which 

 measured 1 1 mm. As compared with the previous stage (Fig. 85), the back has 

 straightened out somewhat, though the lower end of the body is still rolled over. 

 The head has risen and increased considerably in size. Between the end of 

 the region of the hind-brain and the level of the arm the dorsal outline has 

 become slightly concave. This concavity His designated the " Nackengrube. " 

 The first gill cleft, owing to the completed closure of the cervical sinus, is the only 

 one visible externally. It is the anlage of the external auditory meatus. It 

 is separated from the mouth by a prominent mandibular arch. On the cephalic 

 side of the mouth the maxillary process has become- more prominent, but the 



FIG. 84. EMBRYO OF THE PRECEDING FIGURE. X 8 diams. 



two portions of the maxilla do not yet meet in the median line. The primitive 

 segments are still marked externally. The limbs show indications of their tri- 

 partite division, the fore-limb being more advanced than the hind-limb. The 

 division of the digits of the hand is just indicated. The abdomen bulges, out, 

 owing to the growth of the liver. There is a true tail, which is now near its maxi- 

 mum development. The umbilical cord has lengthened and shows the com- 

 mencement of its spiral twisting. The amnion springs from the end of the cord, 

 leaving only a short stretch of the body-stalk between the cord proper and the 

 chorion. The amnion envelops the embryo closely. In embryos slightly older 

 than these the changes in form above mentioned have progressed further. The 



