Warren Harmon Lewis 169 



Embryo XLIII. 



Embryo XLIII measures 16 mm. V. B. and 11 mm. jST. B. It is about 

 six weeks old. Many changes have talcen place during the sixth Aveck. 

 The entire arm has migrated posteriorly, dragging muscles and nerves 

 with it. The brachial plexus has a decided posterior inclination. The 

 skeletal system is much farther advanced and consists for the most 

 part of cartilage; its individual elements are assuming more the adult 

 form. The clavicle now unites the arm and thoracic skeletons. 



The muscular tissues have become more clearly differentiated and ex- 

 cept in the hand are easily distinguished. Muscles, such as the trape- 

 zius, serratus, and pectoral, have spread out into sheets and acquired 

 more their permanent attachments, in the case of the trapezius, latissi- 

 mus and pectorals by migTation or extension of their fibers. 



In the hand, however, we find the interossei still in an undifferentiated 

 condition like that of the deep flexor layer in embryo CIX or the ser- 

 ratus and infrahyoids in embryo CLXIIL' 



The Skeletal System. — The vertebral column. The intervertebral 

 discs are of still more compact tissue than in embryo CIX, but they 

 occupy only about one-fourth height of the segment, while in CIX they 

 occupied nearly one-half the anterior-posterior length of the segment. 

 The body of each vertebra contains a large mass of cartilage, which is 

 continuous with the cartilage in the transverse and neural processes. 

 Indications of the hypochordal brace of Froriep are present in connec- 

 tion with the ventral side of the first three discs. The anterior one is 

 the largest, the others decreasing rapidly in size. 



The ribs are composed of long, slender cartilages, surrounded by a 

 thick perichondrium. This is continuous with the condensed tissue of 

 the tips of the ribs. The tips of the first seven ribs are connected by 

 a narrow strip of condensed tissue which appears to be formed by the 

 turning anteriorly of their tips until they touch the rib above and fuse 

 with it. Thus is formed the' anlage of one-half the sternum on either 

 side some little distance from the median line. There is at present no 

 sign of union of the two halves of the sternal anlagen. The first rib 

 is fused with the median end of the clavicle. The ribs show a marked 

 increase in their lateral convexity, as in embryo CIX there was scarcely 

 any. There are no joint cavities between the ribs and vertebrte. 



The scapula is composed largely of cartilage. It has migrated pos- 

 teriorly so that less than one-half of it lies above the level of the first 

 rib. The whole scapula is larger than in embryo CIX. There is a 

 thick layer of perichondriuui around the cartilage and a considerable 



