George L. Streeter 89 



ventral part of the spinal ganglia it is impossible to determine the 

 presence or absence of fibroblasts belonging to these ganglia. 



The first, or most oral, of the spinal ganglia exhibits special character- 

 istics. It is separated from the second ganglion by an enlarged interval, 

 and it is usually smaller and more slender than the other ganglia. In 

 some cases it consists of a very small clump of cells showing no tendency 

 to unite with its ventral root, or it may be absent completely. On the 

 other hand it may be accompanied by additional ganglionic cell-masses 

 representing the occipital ganglia, found by Froriep, 82, to be constant 

 in the sheep. In one case (Fig. 2) the ganglion is divided ventrally into 

 two clumps of equal size, corresponding to a likewise divided ventral Toot. 

 The more oral of these two may therefore be considered as a persistent 

 occipital nerve and ganglion. 



That part of the ganglion crest which is situated lateral to the after- 

 brain differs from the spinal portion of the crest in that it consists of 

 more loosely arranged cells, and as a whole it is flatter and presents an 

 irregularly triangular profile, which at this period does not exhibit a seg- 

 mental character. That part of it adjacent to the auditory vesicle, and 

 which represents the anlage of the ganglion of the root of the glosso- 

 pharyngeal nerve, is in most cases completely separated from the rest 

 of the crest. It is continued ventralward into the third branchial arch 

 by a looser zone of cells which connects it, just dorsal and caudal to the 

 second gill cleft, with a rounded compact clump of cells forming the 

 future ganglion petrosum. In a similar manner the ganglionic crest of 

 the vagus at its oral end is continuous by means of an ill-defined zone of 

 cells with the anlage of the ganglion nodosum. This ganglion, as' can be 

 seen in Figs. 1, 2, 3, and Plate I, is larger than the ganglion petrosum. 

 It is somewhat spindle-shaped, and owing to the branchial arches it takes 

 a caudal direction ending diffusely beyond the fourth arch. Directly 

 over it lies a patch of thickened epidermis and is apparently adherent to 

 it. A careful search was made here and in the ganglion petrosum for 

 evidence of interchange of cells between the ganglia and the epidermis 

 without success. That these ganglia are not simply ventral projections 

 of the ganglion crest is in a degree indicated by the zone of less well- 

 defined tissue, which at this stage separates them from the crest proper. 



In the ganglion crest, in addition to the cellular elements, there are 

 a few fibres found at the junction of the crest with the neural tube, thus 

 completing the anlages of the glosso-pharyngeal and vagus nerves. A 

 greater fibre development is found in the caudal part of the ganglion 

 crest of the vagus, where inclosed among the cells, as in a sleeve, is found 

 a well-defined bundle of fibres representing the accessory nerve. This 



