12 Development of the Limbs, Body-wall and Back in Man 



The leg-bud arises opposite six myotomes. These usually are the 

 twenty-fourth to the twenty-ninth, corresponding to the five lumbar 

 and 1st sacral myotomes. Caudal to this region arise the remaining 

 sacral and the coccygeal myotomes. 



The myotomes give rise to the dorsal musculature, to the thoraco- 

 abdominal musculature, and to the musculature of the neck, tongue (?) 

 and caudal region. When differentiation of body musculature takes 

 place the distinction between the myotomes becomes lost, and they 

 can no longer be used as landmarks. This occurs by the time the 

 embryo has reached a length of 11 mm. and an age of five weeks. Here- 

 after segmental skeletal structures and spinal ganglia may be used as 

 landmarks. The former present the more stable relative conditions. 



GEXEKAL SUMMAKY OF THE MAIIST EXTERNAL FEATURES NOTED IN THE 



EARLY DEVELOPMENT OF THE SPINAL AXIS, BODY-WALL 



AND THE LIMBS. 



Development of the spinal axis begins in the cervical region. At the 

 end of the second week fourteen myotomes are present (3o, 8c, 3t); at 

 the end of the fourth week, thirty-eight (3o, 8c, 12t, 51, 5s and 5c). Dur- 

 ing the fifth week the myotomes, owing to fusion of their dorsal surfaces, 

 cease to be externally visible. The spinal ganglia, however, give rise 

 to a segmentation externally visible for a somewhat longer period. 



The limbs and body-wall arise in the region where the amnion joins 

 the axis of the embryo. By the end of the second week, at the stage 

 represented in Embryo XIT, the amnion is attached directly to the axis 

 along a line extending from a region anterior to the heart to the caudal 

 extremity (Fig. 1). The amniotic cavity rapidly enlarges. That part 

 of the amnion near the axis of the embryo is carried ventrally so that 

 it closes in the viscera which have previously protruded free into the 

 general coelomic cavity. Finally the amnion reaches the allantoic stalk 

 (or umbilical cord) and becomes attached to this. That part of the 

 amnion extending from the umbilical cord to the axis of the embryo is 

 now known as the menibrana reuniens. It forms the chief covering of 

 the pericardial, pleural and peritoneal cavities. Fig. 2 probably, and 

 Fig. 3 certainly, represent stages in which the viscera are completely 

 inclosed by the menibrana reuniens. 



is possible tliat no great care was taken in determining accurately the number of 

 myotomes in tbis region. In the " Normentafeln " of the chick the number of 

 myotomes pictured in this area varies from seven to ten. Apparently seven in 

 the normal number. 



