TRANSVERSE SECTIONS OF EMBRYO OF 12 MM. 195 



its deeper staining^. It is a section of the cervical sympathetic. Examination 

 with a higher power shows that it consists of somewhat crowded cells, some of 

 which have larger nuclei. These are the neuroblasts. The mesenchymal cells 

 immediately around the anlage are disposed about it in somewhat concentric 

 lines. Between the jugular and the aortic arch is situated the large conspicuous 

 nerve-trunk, N. 10, //, constituted by the united vagus and spinal accessory 

 nerves. Below this double nerve is a blood-vessel, Ve, a branch of the jugu- 

 lar vein which runs to the lower jaw and tongue. The homologies between this 

 vein and those of the adult have not yet been worked out. Returning now to 

 the pharynx, Ph*- on the right side the prolongation of the pharynx to join the 

 fourth cleft can be clearly followed ; on the left side of the embryo, the right of 

 the figure, the fourth cleft, cl. IV, does not display its connection with the 

 pharynx, but is a separate, small epithelial cavity lined by a cylinder epithelium. 

 Underneath the pharynx appears a vertical plate, Tra, formed by the entoderm 

 of the trachea. This plate is thinnest in the middle, somewhat wider toward 

 the top and bottom of the section. It is quite solid, except for a minute cavity 

 at its dorsal end. This minute cavity may be traced from the opening of the 

 glottis through the series of sections down until it becomes connected with the 

 comparatively large cavities of the developing bronchi of the lung. Below the 

 pharyngeal region descends the thick somatopleure, Som, which encloses the 

 pericardial coelom, Cos, in which the heart is lodged. The inner surface of the 

 somatopleure is covered by the thin mesothelium, msth. Of the cardiac struc- 

 tures we note first the section of the main aorta, Ao, and of the pulmonary aorta, 

 P. A, and finally small sections of the uppermost part of the two auricles, Au. d 

 and Au. s. More of the left auricle is included in the section than of the right. 



Section through the Anterior Limbs and Heart. The section figured is much 

 lower in the series than the last and was selected in order to illustrate the ante- 

 rior limb-buds, the ducts of Cuvier, and the heart. The position and shape of 

 the limb-buds are sufficiently shown in figure 96. The section demonstrates ' 

 that the limb-bud is formed chiefly by a dense mass of mesoderm covered by a 

 thin layer of ectoderm. The mesoderm consists of very much crowded cells in 

 which it is very difficult to recognize any distinct differentiations, yet it is prob- 

 able that here are mingled both true mesenchymal cells and cells which originally 

 belonged to the muscle plates, but which have now broken apart and are devel- 

 oping singly into muscleTfibers. In certain amphibia the cells from the muscle 

 plate can be distinguished from the mesenchymal cells of the limb, and what we 

 know of the development of the muscles in amniota confirms t r ,e view that stri- 

 ated muscles and mesenchyma are genetically entirely distinct. No skeletal 

 elements whatever have yet arisen in the limb. We have here a striking illus- 



