498 THE HUMAK EMBRYO. 



twelve thoracic, five lumbar, five sacral, and five caudal. The 

 tail projects freely as a short conical process. 



The fore and hind limbs, LA, LP, are flattened buds, with 

 rounded margins ; they are about as long as they are wide, and 

 show as yet no trace of a division into segments, or into fingers 

 and toes. The outer surface of each limb is its extensor surface ; 

 and the inner, facing the body of the embryo, is the flexor 

 surface. The root of attachment of the fore-limb, or arm, 

 extends from the fifth cervical to the second thoracic somite ; 

 and the attachment of the hind-limb, from the fourth or fifth 

 lumbar somite to the third or fourth sacral. The WolflSan ridge 

 connecting the arm and leg of each side is still present, but is 

 inconspicuous. 



The heart is of great siz6, and its several divisions can be 

 easily recognised through the thin wall of the pericardial cavity. 

 The liver, which is also large, forms a prominent swelling 

 between the heart and the fore-limbs. 



The yolk-sac is about the size of the head and neck of the 

 embryo ; and the yolk-stalk is now long and slender. The 

 inner, or true, amnion is a thin transparent membrane which 

 invests the embryo rather closely ; and the allantoic stalk, 

 which lies to the right of the tail, and to the left of the yolk- 

 stalk, is about 2 mm. long, and rather more than 1 mm. in 

 diameter. 



5. The Fifth Week. 



The fifth week is marked by great increase in size of the 

 whole embryo, and especially of the head ; by further straight- 

 ening of the back, and uplifting of the head ; by the more definite 

 formation of the face ; and by rapid growth of the limbs. 



The cervical flexure, at the junction of the head and body, is 

 still very conspicuous, and throughout the greater part of the fifth 

 week the greatest length of the embryo is, as in the fourth week, 

 a line drawn from this cervical prominence to the sacral curva- 

 ture. At the close of the fifth week, the head becomes lifted up 

 more markedly, and the length of the embryo, about 15 mm., is 

 now represented by a line drawn from the top of the mid- 

 brain to the sacral curve (cf. Pig. 211). 



Throughout the fifth week, the head of the embryo grows 

 rapidly, and by the end of the week it forms, with the neck, about 



