Warren Harmon Lewis 155 



oped is the general arm jjremuscle slieatli, especially its distal portion. 



I have given the name premuscle to various Jiiasses of condensed 

 mesenchyma from which at a later period I believe muscle develops by 

 histogenetic changes of the cells. 



The Skeletal System. — There is no cartilage at this stage. The 

 skeleton is composed of condensed or closely packed mesenchyma which 

 takes a deeper stain than the surrounding tissue. 



The vertebral column consists in the arm region of the intervertebral 

 discs, and their nenral processes which lie in the posterior third of each 

 segment. Between the discs is a loose mesenchyma, the cells of which, 

 as well as those in the disc, have a concentric an-angement about the 

 chorda. 



The ribs spring from the adjacent portions of the disc and neural 

 process. A line of separation is visible. They take a ventrolateral 

 direction into the body wall. The sixth and seventh cervical interver- 

 tebral discs have short rib-like processes., 



In the arm the exact limits of the skeletal strnctures cannot be de- 

 termined as this central core which is easily recognized, shades off into 

 the surrounding mesenchyma, which develops into muscle. The scap- 

 ula is a quadrilateral mass at the level of the fourth and fifth cervical 

 discs. There are no indications of coracoid, acromion or spinous pro- 

 cesses. The scapula is continuous with the humerus, which is a cylin- 

 drical mass occupying the center of the proximal portion of the arm 

 bud. Practically all of it lies at a level anterior to the first rib. At 

 the level of the first rib the humerus is continuous with tlie ulna and 

 radius. There is a slight flexion of the forearm. They are short and 

 thick. The ulna is the larger and is more directly a continuation of 

 the humerus. Partially surrormding the ulna and radius is a plexus 

 of blood-vessels which helps to outline them. The continuation of this 

 plexus is seen in Fig. 8. Both ulna and radius are continuous, with 

 the very ill-defined mass of condensed tissue which lies in the center of 

 the distal end of the arm bud. This rather thin plate composed of cells 

 more closely packed together than those of the surrounding tissue, 

 shows no signs of division into the various elements of the hand. I 

 name it the hand plate. 



The Muscular System. — The muscle plates are fused into a continu- 

 ous column. Indications of segmentation remain. This column lies 

 close and lateral to the neural processes. In the cervical region it ends 

 abruptly at the brachial plexus. In the costal region, however, it ex- 

 tends ventrally into the body wall, between, and partially surrounding 

 the ribs. It ends ventrally beyond the tips of the ribs. The muscle- 



