90 Develoi)mLMit of Occipital Nerves in Human Embryos 



bundle makes its appearance at the level of the fourth, fifth, or sixth 

 cervical segment. It consists of fibres which are contributed to it at 

 irregular intervals by the dorso-lateral border of the neural tube. In- 

 ■ creasing in size it extends forward, running mesially to the ganglion 

 crest until it reaches the crest of the vagus where the cells of the crest 

 form a sheath for it. The tip of this nerve reaches to the dorsal border 

 of the column of cells which extends from the crest to the anlage of the 

 ganglion nodosum. 



The hypoglossal nerve is represented by a row of four or five rootlets, 

 which are in a direct line with those forming the ventral roots of the cer- 

 vical nerves, and resemble them with the exception that they successively 

 taper off smaller as they become more oral. At this stage they have not 

 grown far enough out for coming together to form a common trunk. 

 The rootlets are bundled together in three divisions, between which are 

 situated the first two occipital myotomes. The third myotome lies 

 between the last division of the hypoglossal and the first cervical nerve. 



On looldng back at these embryos of the third week, an important 

 feature is observed in the fact that the main motor elements are indicated 

 at this time by the presence of fibres. We find a few fibres in the roots 

 of the ninth and tenth nerves; the trunk of the eleventh is definitely laid 

 down, also the roots of the twelfth and the ventral roots of the spinal 

 nerves. The sensory elements, however, are only indicated by the celhilar 

 masses of the ganglion crest. 



Embryos of About Four ^Yecli's. 



His Collection, Embryo Br3 6.9 mm. 



Mall Collection, ^o. 2 7.0 mm. 



Gage Collection, Buxton Emljryo 7.0 mm. 



(See Figs. 4, 5 and fi.) 



In this group of embryos the ganglionic crest is increased in size, but 

 aside from a disproportionate increase in fibre elements, it presents much 

 the same appearance as seen in embryos of three weeks. The flattened 

 dorsal border of the spinal crest still forms a continuous bridge along the 

 tops of the primitive ganglia. Cropping out along this bridge numerous 

 fibroblasts are seen attaching themselves to the spinal cord. These are 

 the primitive dorsal nerve roots. Attention is directed to tlie later and 

 slower development of these as compared with the ventral roots. Aside 

 from that seen in the dorsal bridge the ganglia show as yet little or no 

 fibre formation. They consist of cells whose round nuclei, scantly sur- 



