100 Development of Occipital Nerves in Human Embryos 



The principal branches and conininnications of the hypoglossal and 

 cervical nerves may be distinctly traced in embryos of this size. It will 

 be seen that we thus have in this group the completion of the principal 

 features of both the fibre and the cellular elements of the nerves under 

 consideration; but the picture presented by the adult structure varies 

 somewhat from this, owing to a disproportionate growth of some parts 

 over others, and instead of what here appears as a ganglion cell pre- 

 dominance we meet there with a predominance of the fibre elements. 



Emhryos of the Second and Third Month. 



Hertwig Collection, Xo. 161 30.0 mm. 



Mall Collection 65.0 mm. 



(See Plate III.) 



During the second and third months there is a progressive growth of 

 the fibre elements with a corresponding stretching-out of the nerve trunks 

 and rootlets, which results in a greater separation of the ganglion masses 

 from one another. The further growth of the ganglia is not uniform; 

 while the ganglia of the trunk of the ninth nerve, and of the root and 

 trunk of the tenth, and the spinal ganglia continue in their development, 

 the ninth root ganglion and the root ganglia of the eleventh reach at 

 this time a point of development at which they remain stationary. 



In a reconstruction of the left side of the Hertwig embr}-©, No. 161, 

 which is not here reproduced, the noticeable change from the conditions 

 shown in Fig. 12 is in the length and sharper definition of the trunk of 

 -the accessory nerve. On the trunk of this nerve, between the first 

 cervical ganglion and the ganglion jugulare, there are two root ganglia, 

 and as the accessorius trunk joins the vagus there is a third ganglion 

 mass, which, however, is partly fused with the ganglion jugulare. There 

 are also small clumps of cells among the rootlets of the vagus, as well as 

 on some of the central rootlets of the spinal ganglia. 



A dissection of this region in an embryo at the end of the third montli 

 is shown in Plate III. There the ganglia of the root of the ninth and 

 the eleventh nerves present very little enlargement. They can be dis- 

 tinguished from the fibre bundles only by a greater opacity, and appear 

 as white nodes in the roots of the respective nerves. The first cervical 

 ganglion is well developed. An arrest in the growth of this ganglion, 

 similar to that in the ganglia just mentioned, might also be expected. 



As the nerve fibre growth continues, the last trace of these rudimentary 

 ganglia is lost to the naked eye. In order to determine their ultimate fate 

 a series of sections was made throuo-h the structures of this region in an 



