Charles Eussell Bardeen and Warren Harmon Lewis 29 



an ill-defined mass of condensed tissue projecting about one-half 

 the distance from the acromion to the tips of the first rib. The humerus 

 is fairly well defined and is continuous with the scapula as well as the 

 ulna and radius. The elbow bend is well marked. The ulna and radius 

 are farther advanced than in Embryo CLXIII. They end distally in the 

 carpal plate. In the carpal plate indications of formation of the carpal 

 elements are seen. The digits consist of undifferentiated tissue in which 

 are blended skeletal muscle and tendinous elements. The humerus, 

 ulna and radius have a core of hyaline cartilage. The rest of the skele- 

 tal tissue consists of condensed mesenchyme and pre-cartilage. 



In Plate IV are shown the circumflex, radial and musculo-cutaneous 

 nerves. The brachial plexus with portions of the nerves arising from it 

 are seen in Plate V. The plexus is well formed. It has only a slight 

 caudal inclination. The spinal accessory nerve is seen on the median 

 surface of the trapezius. Branches from the 3d and 4th cervical 

 nerves join it. 



Seventh WeeJc. 



By the end of the seventh week most of the structures characteristic 

 of the adult back, body-wall and limbs have appeared. Subsequent 

 development depends in the main upon growth and upon relative shifting 

 of parts. 



The structures of the abdominal wall in a seven-week embryo (XXII, 

 length 20 mm.) are shown in the Figs. F, G, H and I, Plates VI, VII, 

 VIII and IX. The vertebra are composed of embryonic hyaline carti- 

 lage. Each presents a neural and a transverse process on each side. 

 The cartilaginous portions of the vertebrae are shown in the upper 

 thoracic region of Fig. H. The. ribs are likewise composed of em- 

 bryonic cartilage, shown in the same portion of the figure. The cartilage 

 of the ribs is at no time continuous with that of the vertebrae. The ribs 

 and vertebras are surrounded by a dense mesenchyme. This is continu- 

 ous from the ribs to the transverse process of the vertebra. Between 

 the bodies of the vertebrae it forms the intervertebral discs (Fig. I). 

 It is continuous over the surface of the spinal column. From it are 

 developed the ligaments characteristic of the thorax and spinal column 

 together with the perichondrium and periosteum. No thirteenth rib is 

 present in this embryo or in any of the other young embryos we have 

 studied. Fig. 26 shows at the right the portion of the skeleton com- 

 posed of embryonic cartilage, at the left the covering of dense 

 mesenchyme. 



