TRANSVERSE SECTIONS OF EMBRYO OF 12 MM. 189 



formed myotome, My, or anlage of the striated muscular tissue. This tissue is 

 produced from the secondary segments of earlier stages. The cells have now sepa- 

 rated from one another, have lost their distinctly segmental grouping, and have 

 begun to elongate into true muscle-fibers. All that can be distinguished with 

 the low power is the somewhat darker appearance of this part of the section, due 

 to the great crowding of the nuclei. Between the muscular anlage and the noto- 

 chord the section shows a portion of the first cervical nerve, N. cerv. i, and just 

 within this nerve is a small blood-vessel not represented in the figure. There is 

 a similar blood-vessel symmetrically placed on the opposite side. They are the 

 small vertebral arteries. The jugular or anterior cardinal veins, Jug, are large 

 and conspicuous vessels, but despite their size they have merely endothelial walls 

 and there is no condensation of the mesenchymal cells around them, although 

 such a condensation is to take place later to form the anlages of the muscular 

 and connective-tissue coats (media and adventitia) of the adult. On the dor- 

 sal side of the jugular vein and close to it is. a light spot in which can be easily 

 distinguished, with the high power, several more or less distinct bundles of nerve- 

 fibers which are separated from one another by mesenchymal cells. For this 

 reason it is somewhat difficult to recognize this nerve with the low power or to 

 represent it in the figure. On the ventral side of the vein there appears a darkly 

 stained mass, N. 10, the nodosal ganglion of the vagus nerve, and outside of this 

 ganglion is the section of the spinal accessory nerve. Immediately below the tiodo- 

 sal ganglion we have the internal carotid artery, car. in. A little to the inside of 

 the jugular is a small vessel, Ao.D, of great morphological importance. The cor- 

 responding vessel appears on the opposite side. Although very small, this vessel 

 has a distinct coat of condensed mesenchyma around its endothelium. The two 

 vessels are the descending aortae, which have almost completely aborted, and in 

 slightly older specimens will be found to have disappeared altogether. The 

 descending aortae are the longitudinal trunks by which the dorsal ends of the five 

 aortic arches of early stages are connected together. The portion show r n in this 

 section is the part of the descending aorta between the tops of the third and 

 fourth aortic arches. The relations are shown in the reconstruction (Fig. 97). 

 The pharynx, Ph, is narrow in its dorsal ventral diameter, but wide transversely, 

 and offers the very characteristic yoke-shaped figure in the section. The distal 

 portions of the second gill clefts are shown, and they appear disconnected with 

 the pharynx, the connection occurring in sections higher up. Each cleft is some- 

 what slit-like, so that its cavity is an oblique fissure and somewhat parallel in 

 position to the first cleft (Fig. 116). Both the pharynx and the gill clefts are, 

 of course, lined throughout by entoderm, which forms a sharply defined layer 

 crowded everywhere with nuclei, which are of about the same size as those of the 



