Charles Eiissell Barcleen and Warren Harmon Lewis 



15 



extremity of the base ceases to extend over the region of the first lumbar 

 segment. As adult condition is approached, the leg assumes a much 

 more caudal position. 



During the growth of the limbs the body-wall grows forwards at the 

 expense of the membrana reuniens. In the- various figures the ventral 

 line of the body-wall is indicated. The abdominal wall has closed in 

 against the umbilical cord by the time the embryo has reached a length 

 of 6 cm. and an age of about 3 months. 



Having thus considered the more general external features presented 

 in the early development of the limbs and body-wall, let us turn to a 

 consideration of the formation of the main structures within these areas. 





Fig. 16. Cross section 



through the fourth cervical 



segment of Embryo XII. 

 X 55 d. 



II. Internal Structiteal Differentiation. 

 Second Week. 



•-»;4V,.';jv^ ;>■,.-;% «?,*.' -jTi 



Fig. 17. Cross section through the fifth tho- 

 racic segment of embryo LXXVI. x 55 d. The 

 right side of the section passes through the mid- 

 dle, the left side through the posterior third of 

 the segment. 





In an embryo of two weeks, No. XII, Fig. 1, the viscera are attached 

 to the ventral surface of the spinal axis of the embryo, and the amnion 

 is attached to its lateral margin. Fig. 16 shows the conditions which 

 are seen in a transverse section through the fourth cervical segment. 

 The spinal cord is, in this region, a closed tube, the lateral walls of 

 which are composed of five or six layers of epithelial cells. On each side 

 lie the myotomes. These are oval in cross-section, square with rounded 

 corners when viewed from the lateral surface. Each has a well-marked 

 myocoel, surrounded on all sides by four or five layers of epithelial cells. 

 On the left in Fig. 16 the section passes through the centre, at the right 

 it passes through the anterior margin of a myotome. Ventral to the 



