148 The Development of the Arm in Man 



do enter the limbs in the chick. He pictures the ventral end of the 

 myotome entering the limb in company with the nerve and splitting 

 into dorsal and ventral lamellae. Fischel '" believes that in birds and 

 mammals myotome cells mix in the limb bud and give rise to the mus- 

 cles. There is a diffuse entrance of cells from the myotomes but not 

 of myotome buds. In a section of a human embryo of the fourth week 

 he pictures these myotome cells as forming a peripheral layer around 

 the arm bud and even extending into the somatopleure. The rest of 

 the bud comes from the somatopleure. Kollmann '^ pictures in a very 

 diagrammatic manner the downgrowth of the outer lamella of the muscle 

 plate into the arm bud where it lies between the ectoderm and the 

 mesenchymal core. 



In the lower vertebrates it would appear therefore that the limb 

 muscles may arise either from distinct buds of the myotome or they 

 may arise independently of the myotomes from the somatopleure. In 

 the higher vertebrates no distinct myotome buds have been traced into 

 the limbs. Myotome cells are supposed by most observers to enter the 

 limbs and take part in the formation of the muscles. 



The question as to whether in man the muscles of the arm are derived 

 from cells of the myotomes which have migrated into the arm bud at 

 a very early period, I have not been able to determine satisfactorily. 

 Neither am I convinced by the work of Pischel or Kollmann that the 

 myotomes take such a part in the formation of the arm. Their pictures 

 are quite unlike any of the conditions found in the human embryos 

 which I have studied. 



EAELT STAGES OF THE AEM BUD. 



In Embryo CLXIV, 3.5 mm. in length, there are thirteen myotomes. 

 No signs of an arm bud are present. The myotomes are sharply limited 

 and do not give off any cells into the region where the arm bud is soon 

 to sprout. Cells appear to be migrating from the myotome towards the 

 chorda. The somatopleure, hov/ever, shows a proliferation of the cells 

 lining the ccelom. This is very close to the place where the arm bud 

 is soon to appear and lateral to the Wolffian duct and tubules. (See 

 Fig. 1.) 



5io Fischel, Zur Entwickelung der ventralen Rump- und Extremitiiteumusculatur 

 der Vogel und Saugethiere, Morph. Jahr., Bd. XXIII, 1895. 



21 Kollmann, Die Rumpfsegmente menschl. Embryonen von 13 bis 35 Urwirbel, 

 Archiv f. Anat. und Pliys. (Anat. Abtbiel.), 1891. 



