No. 3.] A HUMAN EMBRYO TWENTY-SIX DAYS OLD. 467 



horn of the same. The ganglia are largest in the cervical region, 

 and gradually diminish in size as the tail is approached. The 

 eight cervical nerves are united by anastomoses, which are des- 

 tined to form the cervical and brachial plexuses respectively. 

 The distal ends of the upper eight dorsal nerves are divided into 

 two branches. Beyond these the remaining dorsal, lumbar, and 

 sacral nerves end as a single branch. In all, twenty-nine spinal 

 nerves can be identified ; i.e. eight cervical, twelve dorsal, five 

 lumbar, and four sacral. Beyond this there is a group of my- 

 otomes, which towards the tip of the tail run into one another. 

 In this region the dorsal ganglia are not fully separated from 

 the spinal cord ; in fact, the whole seems yet to be blended with 

 the ectoderm. 



Sympathetic Nerves. 



Onodi 1 has shown quite conclusively that the sympathetic 

 nerves arise from the spinal. In our embryo, although there are 

 as yet no sympathetic ganglia, there are marked branches from 

 the first six dorsal nerves extending directly towards the chorda. 

 These branches, without their ganglia, are spoken of by His for 

 a human embryo 7 mm. long, in his last communication. 2 It 

 will be seen from the reconstruction that all the sympathetic 

 nerves arise from the oral branches of the spinal nerves. Three 

 are in front of the cceliac axis, and three behind. From the 

 study of other mammalian embryos I find that at about this 

 stage the sympathetic nerves become encircled about the cceliac 

 axis ; and as this vessel moves aboralwards, the successive 

 branches are entangled, and in this way form the splanchnic 

 nerve. In the adult the cceliac axis is back of the twelfth 

 dorsal vertebra, and the solar plexus encircles it. This plexus 

 communicates by means of the great splanchnic nerve with the 

 fifth to the tenth dorsal nerves. Under favorable conditions it 

 may be traced to the third, second, or even the first dorsal. 3 

 Now in the various stages of development, the cceliac axis is 

 successively opposite the various dorsal vertebrae ; and as it 

 moves backwards it carries with it these sympathetic twigs from 

 the spinal nerves, which all unite to form the splanchnic. 



1 Onodi, Arch.f. mik. Anal., Bd. XXVI., 1886. 



2 His u. Braune's Archiv, Supplement-Band, 1890. 

 8 Beck, Phil. Trans., 1846. 



