HuBER, Sympathetic Nervous System. 8i 



motor neurons, the dendrites present in the fully developed mo- 

 tor neurons budding from the cell bodies at a later stage in their 

 development. The other germ cells, that is those not develop- 

 ing into motor neurons, form intra-medullary neurons. 



While these developmental changes are going on in the 

 neural canal, similar changes are seen in that portion of the ec- 

 toderm just outside of the canal. In this region, even before 

 the medullary groove has become a canal, germinal cells, like 

 those found in the sides of the groove, wander out into the an- 

 gular space between the ectoderm and closing neural canal, to 

 form the neural crests. These neural crests, composed of 

 germinal cells, segment, a group forming opposite each meso- 

 blastic somite. The cells in these segments wander to a posi- 

 tion between the neural canal and the respective somites, and 

 form the anlagen for the spinal ganglia. The cells have become 

 bipolar and are known as gangioblasts (Lenhossek), one of the 

 processes growing into the dorsal portion of the developing neu- 

 ral canal, forming the posterior or sensory root, the other to- 

 ward the periphery, joining the developing anterior root and 

 forming the sensory fibers of peripheral nerves. The bipolar 

 cells become unipolar, with "T-shaped" processes as found in 

 the fully developed- spinal ganglia, either by a fusion of a short 

 portion of the two processes or, as Lenhossek has suggested, by 

 drawing away of the cell body from the two processes, the ex- 

 tended and attenuated portion of the cell-body forming the ver- 

 tical arm of the " T. " 



I have in this very brief account thus hastily traced the 

 development of motor, sensory and intra-medullary neurons, 

 and a moment's reflection will show that only the sympathetic 

 system, and some of the special sense organs are still to be 

 accounted for. 



The development of the sympathetic system begins some- 

 what later than does the cerebro-spinal system. 



His (lo) states that, in the human embryo of 7 mm., there 

 is as yet no evidence of the sympathetic system, while the me- 

 dullary canal is completely closed, and the ganglia of all the 

 spinal nerves as well as the anterior roots are to be seen. In a 



