FRONTAL SECTIONS OF EMBRYO OF 12 MM. 215 



has been cut transversely. Below it and near the pharynx is the small noto- 

 chord, which, however, can only be clearly recognized with the higher power, and 

 is, therefore, not represented in this or the preceding figure. The otocyst is a 

 large epithelial vesicle with three well-marked divisions: first, the common 

 chamber, S. c, out of which the three semicircular canals are to be differentiated. 

 Second, a slender canal, D.e, which one easily identifies as the anlage of the ductus 

 endolymphaticus. It lies between the semicircular canal and the wall of the 

 medulla oblongata. Third, the long, curving, but not spiral cochlea. The com- 

 mon chamber formed by the union of these divisions is later subdivided to form 

 the upper utriculus and lower sacculus. Outside the cochlea lies the cross- 

 section of the jugular vein, just below which is the section of the motor portion, 

 Fac. m, of the facial nerve. The sensory portion of the facial nerve at this stage 

 is much smaller, and runs only a short distance downward from the geniculate 

 ganglion and is entirely separate from the motor portion. The morphological 

 constitution of the facial nerve is still very obscure, and a satisfactory account 

 of its development is, for the present, impossible. 



Section through the Dorsal Vertehrce (Fig. 1 28). — Owing to the curvature of the 

 embryo the spinal cord is cut twice ; once, Sp. c' , toward the head end of the em- 

 bryo, and again, Sp. c", lower down toward the tail end. Alongside the sections 

 of the spinal cord appear the large, darkly stained masses of the ganglia, G. The 

 'section also passes through the bases of the anterior limbs, A. L,m one of which 

 we can see one of the branches, A^. br, of the brachial plexus. Between the two 

 pieces of the spinal cord of the section the plane passes on the ventral side of the 

 spinal cord and shows the series of vertebral formations, together with the nerve- 

 roots, A^'', the intersegmental arteries, A. i. s, and the segmental veins, small 

 vessels which lie close to the intersegmental arteries. The nerves are sections of 

 the dorsal root below the ganglia. Each nerve has a distinct outline and is partly 

 penetrated by ingrowing mesenchymal cells which subdivide the nerve into 

 rounded fiber bundles. In each bundle the nerve-fibers appear as fine dots, 

 which, however, by the use of the fine adjustment can be followed up and down 

 through the section, and thus identified as fibers. The single fibers are more or 

 less isolated from one another, and between them are delicate threads, the nature 

 of which is not known. Between the adjacent rounded bundles of fibers there is 

 often a distinct space. The vertebral anlages, Vert, are formed entirely from 

 condensed mesenchyma, and therefore stand out somewhat conspicuously in the 

 section, owing to their darker staining. Each anlage is bow-shaped, the con- 

 cavity of the bow facing toward the tail of the embryo. The ends of the bow pass 

 behind the nerve-trunk of the segment to which the vertebral anlage belongs. 

 The anlages extend completely across the median line, and by following through 



