Charles Eussell Bardeen and Warren Harmon Lewis 13 



During the second half of the third week the membrana reuniens 

 becomes markedly thickened along its line of attachment between, 

 usually, the fourth and twenty-sixth spinal segments. This thickening 

 is known as the WoMan ridge. In Figs. 2-7 the ventral margin of this 

 ridge is emphasized by a heavy line. Opposite the fifth to the ninth and 

 the twenty-first to the twenty-sixth spinal myotomes this thickening is 

 especially marked. The two latter areas represent the inception of the 

 limb-buds; the intervening area represents the rudiment of the lateral 

 body-wall. These three areas first become well marked toward the end 

 of the third week (Figs. 2-3). 



The limb-buds increase very rapidly in size. At first the limb-buds 

 extend directly laterally from the Wolffian ridge (Figs. 2 and 3), but as 

 development proceeds they project more and more in a caudal direction 

 (Figs. 4, 5 and 6). Meanwhile two distinct divisions appear in each limb- 

 bud. The part which arises directly from the Wolffian ridge we may 

 call the basal division. This portion continues to grow in the ventro- 

 lateral direction taken by the limb-bud originally. Beyond this basal 

 part the limb-bud bends in a ventro-median direction. The part of the 

 limb-bud beyond the bend we may call the " distal part " of the limb- 

 bud. For some time the basal portion of the limb-bud continues to 

 grow in a ventro-lateral direction, while the distal part grows in a 

 ventro-median direction. The limb as a whole meanwhile points dis- 

 tinctly in a caudal direction. 



Dorsally the base of the limb becomes continuous with the dorsal 

 margin of the Wolffian ridge, ventrally with its ventral margin. The 

 dorso-lateral surface of the base of the limb-bud is therefore extensive; 

 the ventro-median surface is small in area. Owing to the caudal direc- 

 tion assumed by the limb, the anterior (cephalic) surface of the base is 

 extensive, while the posterior (caudal) surface is limited in extent. 



The distal part of each limb-bud is flattened so that it presents median 

 and lateral surfaces and anterior (cephalic), ventral and posterior 

 (caudal) margins. A constriction may be seen immediately beyond the 

 region where the distal flattened portion of the limb joins the thicker 

 rounded base. In Fig. 8 the constriction is emphasized by heavy lines. 

 In the arm it is seen as one looks at the limb directly from the side. In 

 the leg, which is here slightly twisted, one may see the constriction as it 

 appears when the limb is viewed on the anterior margin. The con- 

 striction is just beginning to appear in the leg-bud shown in Fig. B, 

 Plate II, and in Fig. C, Plate III. 



From the basal ^portion of each limb-bud are developed the limb- 

 girdle (shoulder and pelvic-girdles), and the upper limb (upper arm and 



