TRANSVERSE SECTIONS OF EMBRYO OF 12 MM. 279 



and only a portion of the right arch will persist to form a portion of the stem of 

 the pulmonary artery. The figure indicates the manner in which these aortic arches 

 pass up from the heart below on either side of the pharynx. A little above the 

 aortic arch on either side may be seen a small, round spot, Sym, which is 

 somewhat conspicuous on account of 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 neuro- 

 blasts. The mesenchymal cells immediately around the anlage are disposed about it in 

 somewhat concentric lines. Between the cardinal vein and the aortic arch is situated 

 the large, conspicuous nerve-trunk, N. 10,1*1, constituted by the united vagus and 

 spinal accessory nerves. Below this double nerve is a blood-vessel, Ve, a branch 

 of the cardinal vein. This vessel drains the tongue and facial region of the em- 

 bryo. It is homologous with the inferior jugular vein of lower vertebrates, and 

 in mammals gives rise to the lingual and facial veins of the adult, and in some 

 species forms the external jugular, but the human external jugular is a secondary 

 anastomosis between the linguo-facial and the junction of the internal jugular and 

 subclavian veins. 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 

 solid, except for a minute cavity at its dorsal end. This minute ity 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, Cce, in which the heart is lodged. The inner 

 surface of the somatopleure is covered by the thin mesothelium, msth. Of the 

 cardiac structures 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 

 of the right. 



Section through the Anterior Limbs and Heart (Fig. 196). The section figured is 

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

 anterior limb-buds, the common cardinals, and the heart. The position and shape 

 of the limb-buds are sufficiently shown in figure 166.' 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 fells in which it 

 is very difficult to recognize any distinct differentiations, yet it is probable that 

 here are mingled both true mesenrhymal cells and cells which originally belonged to 





