Charles Eussell Bardeen and Warren Harmon Lewis 3 



sketches of the embryos indicated. On Plate I the photographs utilized 

 are reproduced. On Plates II to IX are represented several typical 

 stages in the general development of the body-wall and limbs. Figs. 

 A and B, Plate II, are drawn from wax-plate reconstrnctions. Figs. C 

 to E, Plates III to Y, are based, in the main, npon a reconstrnction of 

 the regions of the arm, abdomen and leg of embryos CLXIII and CIX, 

 and npon excellent photographs. Figs. F to I, Plates VI to IX, are 

 based npon wax-plate reconstrnctions of Embryo XXII. 



Fig. 1. 

 X about 10 d. 



Fisr. 2. 



Fig. 3. Fig. 4. 



X about .5 d. 



Fig. .5. 



i:mbryo XII. 



The development of the neuro-muscular apparatus begins in the 

 human embryo in the cervical region. In Fig. 1 is represented Embryo 

 XII, 2.1 mm. in length and about two weeks of age. The axis of the 

 embryo is curved in a semicircle about the heart and the umbilical vesicle. 

 The axis contains neural tube, notochord, myotomes, dorsal-aortas, and 

 mesenchyme (see Fig. 16). There are fourteen myotomes on each side. 

 Mall considers three of them occipital, eight cervical, and three thoracic. 

 The first cervical and the first thoracic myotomes are numbered " 1 " in 

 Fig. 1. Caudal to the fourteenth myotome, an unsegmented band of 

 tissue extends along each side of the spinal cord. The neural tube is 

 open dorsally anterior to the fourth and posterior to the fourteenth 

 myotome. Opposite the twelfth myotome a solid band of cells, the 

 "' neurenteric canal," unites spinal-cord and entoderm. The notochord 

 extends from a point opposite the cephalic margin of the heart to the 

 region of the neurenteric canal. The dorsal aortae run a course parallel 

 with the notochord, but extend further than the myotomes caudally. 

 A considerable amount of mesenchyme is formed at the cephalic ex- 

 tremity of the axis of the body, in the region of the heart, but toward 

 the caudal extremity little exists. The heart and the pericardial and 

 pleural cavities are developed in the cephalic regi'on of the wall of the 

 umbilical vesicle. Between the region of the heart and the neural 



