Warren Harmon Lewis 



149 



Embryo XII," 2.1 nun. in length, has fourteen myotomes and is slight- 

 ly older than CLXIV. The first definite signs of an arm bud are here 

 noticed by a slight swelling ventrolateral to the myotomes in the lower 

 cervical region. Its position is seen in Fig. 2. The origin of the cells 

 which cause this swelling I am not able to determine, though there are 

 suspicious looking processes from the myotomes. No spinal nerves are 

 present. 



Embryo LXXYI is 4.5 mm. in length and about three weeks old. 

 Between embryos XII and LXXVI there is quite a gap. There are 







' IVolffian duct.'^ '( 











^yi-^^s-!^ 



Fig. 1. Cross section through the eighth mj'otome of embryo CLXIV. 

 X 100 diameters. 



35 myotomes. The arm bud is quite large and filled with uniformly 

 and closely packed cells whose nuclei take a deep stain with the 

 alum carmine. A few thin-walled blood-vessels are scattered here and 

 there. The base of the arm bud lies opposite the fifth cervical to the 



^- Dr. Mall considers embryo CLXIV slightly older than embryo XII. The greater 

 length of CLXIV he accounts for by a straightening of the body of the embryo 

 through mechanical injury to the ovum. See Mall, On the development of the 

 human diaphragm, The Johns Hop. Hosp. Bui., Vol. XII, 1901, p. 160. 



