10 Development of the LimbS;, Body-wall and Back in Man 



RELATIONS OF THE LIMBS AND BODY-WALL TO THE SPINAL SEGMENTS. 



In noting the main features of development in early human embryos, 

 the most convenient landmarks for describing relative positions of 

 structures are the myotomes. By the end of the second week of em- 

 bryonic development about fourteen myotomes have been distinctly 

 differentiated in the human embryo (see Fig. 1). The formation of 

 myotomes continues until by the end of the fourth week about thirty- 

 eight have been differentiated (see Embryo II, Fig. 6). Anterior to the 

 eight cervical myotomes, three occipital myotomes seem usually to be 

 developed; posterior to the cervical region, twelve thoracic, five lumbar, 

 five sacral, and five to six coccygeal myotomes are formed. The di- 

 vision of the myotomes into occipital, cervical, thoracic, luinbar, sacral 

 and coccygeal groups depends upon the nerves and skeletal structures 

 related to the body segments in which the myotomes lie. 



The occipital myotomes are transient structures developed in con- 

 junction with roots of the Twelfth Cranial Nerve. Before the spinal 

 nerves have appeared they cannot with certainty be distinguished. No 

 account of occipital myotomes is given in the description of the embryos 

 pictured in the His Atlas. In the Mall embryos, LXXVI. CXLVIII 



To the His embryo A, Fig. 7, correspond : 



His's embryo Pr, length 10 mm., Atlas, Taf. XIII, Fig. 4, and Taf. X, Fig. 10. 



His's embryo B, length 7 mm.. Atlas, Taf. I, Fig. 1. 

 To No. CLXIII, Fig. 8, correspond: 



Kollmann's 10.3 mm. embryo, Arch, of Anat. and Phys., 1891, Taf. Ill, Fig. 1. 



Ruge's 9.1 mm. embryo, His's Atlas, Taf. XIII, Fig. 5, and Taf. X, Fig. 12. 

 To the His embryo Br, Fig. 9, corresponds : 



Keibel's embryo, H. S. Bui., length 11. .5 mm.. Arch, fiir Anat. und Phys., 1891, 

 Taf. XIX, Fig. 13a. 

 To No. CIX, Fig. 10, corresponds : 

 ' His's embryo S, length 13..5 mm.. Atlas, Taf. XIII, Fig. 7, and Taf. X, Fig. 16. 

 To No. CLXXV, Fig. 11, corresponds: 



His's embryo^Schj, length 13.8 mm., Atlas, Taf. XIV, Fig. 3, and Taf. X, Fig. 18. 

 To No. CVI, Fig. 13, correspond : 



His's embryo Br^, length 13.3 mm.. Atlas, Taf. XIV, Fig. 1. 



Ruge's 13.6 mm., embryo, His Atlas, Taf. XIV, Fig. 4, and Taf. X, Fig. 19. 

 To No. CLXVII, Fig. 13, corresponds : 



His's embryo Pr, length 14..5 mm.. Atlas, Taf. XIV, Fig. .5, and Taf. X, Fig. 30. 

 To No. XLIII, Fig. 14 corresponds : 



His's embyro 8^, length 1.5..5 mm., Atlas, Taf. X, Fig 31. 

 To No. XXII, Fig. 1.5 correspond : 



His's embryo XCI, length 16 mm., Atlas, Taf. X, Fig. 33. 



His's embryo Ltj, length 17..5 mm., Atlas, Taf. X, Fig. 33. 



His's embryo Z. W., Atlas, Taf. X, Fig. 34. 



