PLATE IV. 



Figure D. Magnification about 12 d. 



Lateral view of Embryo CIX; length, 11 mm.; age, about 5 weeks. The 

 areas from which the skin has been removed are drawn from wax-plate 

 reconstructions, the remaining portions are drawn partly from photographs, 

 partly from an embryo of corresponding age. The arches and transverse 

 processes of the 4th to 8th cervical vertebrae have been exposed by the 

 removal of the dorsal musculature in that region. The embryonic cartilage 

 of which these structures are formed has been exposed by the removal of 

 the perichondrial mesenchyme. The dorsal musculature has likewdse been 

 removed from the 5th lumbar and first three sacral segments. In this 

 region, however, is shown the dense mesenchyme which incloses the car- 

 tilaginous portions of the spinal column. 



The heads of the first three ribs may be seen median to the transverse 

 processes of the first three thoracic vertebrae. The third to the eleventh 

 ribs may be seen through the lateral musculature of the body-wall. 



The dorsal musculature is distinctly separated from the ventro-lateral. 

 In the thoracic region little evidence remains of segmentation in the dorsal 

 musculature. In the lumbar, sacral and coccygeal regions myomeric struc- 

 ture is still visible. The ventro-lateral musculature, which has developed 

 from processes from the twelve thoracic myotomes, is beginning to assume 

 a differentiation into the muscles characteristic of the thorax and abdomen. 



The dorsal divisions of the spinal nerves are shown in the regions where 

 the vertebrae are exposed. The lateral branches of the ventral divisions are 

 shown in the thoracic region. 



In the region of the anterior limb superficial tissues have been removed 

 so as to expose the main structural features. Near the spinal column the 

 trapezius and serratus anticus muscles are shown, the former being repre- 

 sented as semi-transparent. From the shoulder the greater portion of the 

 deltoid muscle has been removed, from the upper arm the greater portion 

 of the triceps, and from the forearm the greater portion of the extensor 

 digitorum communis. 



In the region of the leg the more superficial tissue has been removed so 

 as to expose the skeletal, muscular, nervous and vascular apparatus. 



The skeleton consists of hip-bone, femur, tibia and fibula, which are com- 

 posed of embryonic pre-cartilage covered by a dense mesenchyme, and of 

 a dense mass of tissue which represents the anlage of the ankle and foot. 



The five lumbar and the first three sacral nerves enter into the formation 

 of the lumbo-sacral plexus. From this arise the femoral nerve, which 

 enters a mass of tissue that represents the extensor muscles of the thigh; 

 the peroneal nerve, which gives off gluteal branches to the gluteal muscle 

 mass, a posterior cutaneous branch, branches to the extensor musculature 

 of the foot, and a peroneal branch, which extends a short distance along 

 the fibula. The peroneal musculature has not yet become differentiated 

 from the mesenchyme. At the posterior extremity of the plexus the pudic 

 nerve may be seen. 



The border vein empties into the femoral and sciatic veins. The sciatic 

 artery is shown terminating in the extensor musculature of the foot. 



