George L. Streeter 93 



however, is well developed. Cases of this kind have in the adult the 

 appearance of entire absence of this ganglion, a point which will be taken 

 np later. 



The caudal portion of the ganglion crest of the after-brain is longer 

 and more slender than in the previous stage. The accessory nerve is still 

 ensheathed by the cells of the crest. As it extends forward it turns the 

 curve on the back of the trunk of the vagus, and then freeing itself from 

 the vagus it extends a short distance lateralward and ends abruptly in a 

 mass of condensed mesoderm, the anlage of the m. sterno-cleido-mastoi- 

 deus. The oral end of the ganglion crest of the vagus is connected with 

 the ganglion nodosum (it will be observed that this ganglion is not con- 

 sidered as simply a part of the vagus crest, which is because of the 

 apparent independence of the two seen in three week embryos) by a com- 

 pact mass of cells, among which are found some fibres. From the 

 ganglion nodosum there arise two distinct fibre bundles, ventrally the 

 superior laryngeal, and ventro-caudally the main trunk of the vagus, 

 around which winds the hypoglossal nerve. 



The rootlets of the hypoglossal nerve unite and form a stem, as in 

 the adult, which seems to be joined by fibres from the first and second 

 cervical nerves at the point where it bends upward to reach the anlage of 

 the tongue. • The descending branch of the hypoglossal can be identified 

 on the right side of Embryo Br3 as a short bud at the point where the 

 nerve crosses the main trunk of the vagus. Thus the descendens hypo- 

 glossi develops in this case simultaneously with the appearance of 

 anastomoses between that nerve and the upper cervical nerves. 



The root of the glosso-pharyngeal nerve is definitely connected with 

 the ganglion petrosum by a fibro-cellular mass. Ventral to the ganglion 

 the trunk of the nerve is represented by a fibre strand extending into the 

 third branchial arch. In Fig. 6 is represented the area on both the 

 petrosal and nodosal ganglia which still remains attached to the overlying 

 thickened epidermis. In Embryo Br3, on both sides of the embryo, the 

 ganglia nodosum and petrosum appear to fuse. 



On review of this group it is seen that at the end of the fourth week 

 the ganglion crest is not yet entirely differentiated. We find laid out, 

 however, the roots, the trunks, and the ganglia of the trunks of the ninth 

 and tenth cranial nerves. Further, all the elements of the eleventh and 

 twelfth nerves are present, and the dorsal and ventral roots and the 

 plexuses of the spinal nerves. 



