134 A HUMAN EMBRYO OF TWENTY-FOUR PAIRS OF SOMITES. 



which surround them. They are connected with the wall of the brain by strands 

 of cells; definite fibers are apparently just beginning to form. Distally, ganglion 

 cells and fibers can be traced for some little distance, the trigeminal nerve passing 

 toward the maxillary and mandibular arches and the acustico-facial into the 

 dorsal portion of the second arch. 



In addition to the trigeminal and acustico-facial nerves, there is found just 

 behind the auditory vesicle and near the brain-wall a small cluster of undiffer- 

 entiated cells. These are found in the region of the sixth neuromere and very 

 probably represent the ganglion of the glosso-pharyngeal nerve. Opposite the 

 seventh neuromere is another similar group of cells, the probable beginning of the 

 vagus nerve. 



GANGLIONIC CREST. 



Indistinctly connected to the vagus ganglion and extending down beyond the 

 ventral bend in the back of the embryo, a ganglionic crest is discernible. It is 

 not distinct, however, for its cells so closely resemble those of other adjacent 

 tissues that they can not always be identified with certainty. It is largely due 

 to the position of its cells, i. e., between the medullary tube and the somites, and 

 to the arrangement of its nuclei, that the presence of the ganglionic crest can be 

 detected. 



Streeter 40 has described the neural crest of a 4 mm. embryo as follows: 



"This structure (the ganglionic crest) can be seen in the 4 mm. embryo as a flattened 

 cellular band which extends caudalward from the auditory vesicle along the lateral wall 

 of the neural tube to its extreme tip .... That part of the crest which corresponds 

 to the spinal cord is characterized at this time by segmental incisures along its ventral 

 border. The dorsal border of the crest remains intact until the appearance of the dorsal 

 rootlets, in the meantime constituting a cellular bridge connecting the more ventral gang- 

 lionic clumps." 



In my specimen the ganglionic crest likewise forms a cellular band. Seg- 

 mental incisures along the ventral border are indicated in certain regions. The 

 crest, however, is apparently not so far developed as shown by Streeter. Caudally 

 its development is not so far advanced as toward the head. So indistinct and 

 uncertain are its outlines, as seen in sections, that I have made no attempt to 

 represent it graphically. 



Lenhossek 25 has shown the ganglionic crest in a formative stage in a human 

 embryo of 13 segments. Its cells are described as arising from the roof of the 

 medullary tube. Such pictures as Lenhossek has shown I have been able to find 

 in my specimen only in the caudal portion of the body, where the roof of the 

 medullary tube is relatively thick; there, as seen in cross-section, the developing 

 ganglionic crest appears as a cap lying on the roof of the medullary tube; it is 

 composed of closely packed cells with nuclei of about the same size as those of 

 the remainder of the tube. 



