TRANSVERSE SECTIONS OF EMBRYO OF 12 MM. 193 



G. nod, the ganglion nodosum of the vagus nerve. Immediately above it is 

 the section of the jugular or anterior cardinal vein, Jug. Between the ganglion 

 and the vein is a bundle of nerve-fibers representing the twelfth or hypo- 

 glossal nerve. Close to the ganglion on its outer side is the section of the 

 spinal accessory nerve, A^. //, which reappears again in the section below the 

 pharynx, at A^. 12. The reason for this double appearance of the hypoglos- 

 sal nerve may be seen readily by examination of the reconstruction (Fig. 105). 

 A little above the jugular vein is the section of the first cervical nerve, A^. cerv. i, 

 laterad from which is the external branch, R.ex p, of the spinal accessory nerve. 

 This branch in the adult innervates the sternocleidomastoid and trapezius 

 muscles. 



The lower part of the figure represents the section of the head and shows 

 the two nasal fossae, Na, closed toward the mouth side by the olfactory plate, 

 01. pi, the epithelial membrane somewhat resembling the closing plate of a gill 

 cleft, but it is formed by a fusion of the ectoderm on the two sides of the opening 

 of the nasal pits. When the nasal pits are first formed, they are open throughout 

 their whole extent. The formation of the olfactory plate is the first step toward 

 the separation of the two nasal cavities from the oral cavity. In later stages this 

 plate disappears, and its forward portion is replaced by mesenchyma, so that the 

 separation of the nasal and oral cavities is permanent, but the posterior portion 

 of the membrane becomes first very thin, and finally disappears altogether, thus 

 establishing a secondary connection between the nose and mouth for each nasal 

 chamber, thus leading to the development of the internal choanae. On the dorsal 

 side of the nasal pits (below in Fig. 118), the cerebral hemispheres are cut sepa- 

 rately, their darkly stained walls bounding on each side the large lateral ventricle. 



Section through the Fourth Gill Cleft. — Of the entodermal gill clefts or 

 pouches the fourth is by far the smallest, and as it appears in sections (Fig. 1 19, 

 cl. IV) is inconspicuous. The section figured differs by two striking features 

 from those of the series above described : first, because the head is no longer 

 included; and, second, because the cardiac structures are beginning to show. 

 On the dorsal side the spinal cord is cut at the level of the gangUon, G.j, of the 

 third cervical cleft. The dorsal root of the ganglion joining the spinal cord, Sp. 

 c, is shown on both sides of the section, and the nerve itself also appears, being 

 best shown on the left side of the embryo, where a short piece, R. D. j, of the 

 ramus dorsaHs is included and a much longer piece, R. V. j, of the ramus ven- 

 tralis. Just inside of the nerve at the level of the notochord, Nch, is the cross- 

 section of the vertebral artery. On the right side of the embryo the section 

 passes through a portion of the second cervical nerve, N. cerv. 2. The jugular 

 vein. Jug, is a very large vessel. Close to its ventral wall appear a few fibers 

 13 



