106 Dcvolopmcnt of Oeoipitiil Norvos in TTiiinan l^ml)rvos 



size, is niodided in ronii by llii^ (IcvcIopnKMit ol' numerous rootlets attach- 

 ing it to tlie neural tube, and by an irregular clumping together of the 

 cells of the crest, forming ganglion masses along the main trunk of the 

 accessorius, which now lies at the ventral border of the crest. Tlie root- 

 lets which are developed at this time are in part sensory, as is evidenced 

 by comparison with the dorsal spinal roots. The division of the crest 

 into ganglion masses is accompanied by a rapid development of fibres 

 between its cells, and it is probable that the growth of these fibres is 

 the cause which spreads the cell masses apart into separate clumps. 

 Such a separation into clumps radically differs from true segmentation, 

 the latter does not seem to occur here. As the fibre growth continues 

 the ganglion masses become more and more separated, and finally the 

 crest becomes completely converted into a series of discrete ganglia 

 (Figs. 10 and 11). The most oral one is the largest and forms the vagus 

 root ganglion, the ganglion jugulare. Caudal to this, successively dimin- 

 ishing in size, is a chain of three or four accessory root ganglia, which 

 extend backward along the accessory trunk until they meet the cervical 

 ganglion series. In the vagus complex the ganglia diminish in 

 size in the caudal direction, while in the spinal series the reduc- 

 t?on in size is in the oral direction ; this fact enables one to distin- 

 guish between accessorius root ganglia and precervical (Froriep) ganglia. 

 Those ganglion masses found adherent to the accessorius between the 

 first and second, and the second and third cervical ganglia, as in Figs. 10, 

 11, and 12, are developed from the spinal crest, and represent nodules 

 derived from the spinal ganglia and which have become separated off. 

 Formed similarly to these, are found isolated masses on tlie rootlets of the • 

 jugular ganglion. 



The vagus division of the complex at this stage (embryos of 11.0 mm.) 

 therefore consists of mixed motor and sensory rootlets, the ganglion Jugu- 

 lare, and the nerve trunk on which is situated the ganglion nodosum, 

 giving off a lar}aigeal branch as well as communicating branches to the 

 ganglion petrosum. The accessory division begins at the third or fourth 

 cervical segment. Its trunk runs median to the dorsal roots, except at the 

 first cervical where it may be lateral. It is attached to the neural tube by 

 mixed rootlets, on which are found a varying number of ganglia. The 

 trunk after an arched course joins the vagus division, but the greater 

 portion of it soon leaves the vagus and extends to the shoulder region 

 and supplies the sterno-cleido-mastoid and trapezius muscles. 



The essential features of embryos of the fifth and six week, Figs. 11 

 and 12, will be observed to be the same as in older embryos and in the 

 adult, Plate III and Fig. 13. The existing difference may be accounted 



