TRANSVERSE SECTIONS OF EMBRYO OF 12 MM. 277 



in the median line for a considerable distance, making a figure which, in the 

 section, is shaped somewhat like an inverted spear-head. In the center of the 

 section appears a small cavity. Farther down toward the lungs we have only an 

 epithelial plate with no cavity observable in it (Fig. 195, Tra), the entoderm of 

 the trachea at this stage forming a solid cord. Ventrad from the trachea, in 

 the median region and between the two aortic arches, is a small, irregular, deeply 

 stained mass of cells, Thyr, the anlage of the thyroid gland. These cells are ento- 

 dermal, the anlage having been developed by a downgrowth of the epithelium of 

 the floor of the pharynx, although at the present stage the original connection with 

 the pharynx has been lost. The anlage is now isolated from its parent germ-layer 

 and is imbedded in mesenchyma. It is solid, for the cavities of the thyroid follicles 

 are not developed until considerably later. Just above the third gill-cleft may 

 be seen a large, darkly stained mass, G.nod, the ganglion nodosum of the vagus 

 nerve. Immediately above it is a section of the anterior cardinal vein, Card. 

 Between the ganglion and the vein is a bundle of nerve-fibers representing the 

 twelfth or hypoglossal nerve, which reappears again in the section below the 

 pharynx, at N.I2. The reason for this double appearance of the hypoglossal 

 nerve may be seen readily by examination of the reconstruction (Fig. 178). Close 

 to the ganglion on its outer side is the section of the spinal accessory nerve, N.n. 

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

 laterad from which is the external branch, R.ex.n, 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 pits, Na, closed toward the mouth side by the olfactory plate, Ol.pl, 

 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 y plate is the first step toward 



the separation of the two nasal cavities from i ravity. In later stages this 



plate disappears, and its forward portion is replace m:hy ni, so that the 



separation of the nasal and oral cavities is permanent posterior portion 



of the membrane becomes first very thin, and finally di^ r '-tiier, *'.,. - 



establishing a secondary connection between the nose and mo, 

 chamber. On the dorsal side of the nasal pits (below in Fig. 1(4.. 

 hemispheres are cut separately, their darkly stained walls bounding on <_ 

 the large lateral ventricle, L.V. 



Section through the Fourth Gill-Cleft (Fig. 195). Of the entodermal gill-cleft? 

 or -pouches the fourth is by far the smallest, and as it appears in sections (rl.l \' : 

 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 ievei of U.L ganglion, (7. 3, of the third cervical nerve. 



