6 



MORPHOLOGY OF THE 



The posterior or sensitive nerve roots have a totally 

 different history. 



Before the closing of the medullar}' groove, certain 

 large and well-defined ectodermic cells are easih' distinguish- 



Fig. 3. 



(Minot. 344:.) Sections through the cervical part of the medulla of a human 

 embryo, with thirteen segments. in front of the segments where the medullary 

 groove is Avidely open. B. a little further back, where the medullary groove is just 

 closing. (7. at the level of the third segment. Ec. ectoderm. Gl^ ganglionic 

 anlage. Mch medullary wall. Mes. mesoderm. After Leuhossek. 



able at the point where the skin layer of the ectoderm is 

 about to separate from the closing medullary groove. These 

 cells leave the medullary wall, and wander external to it 

 within the mesoderm, and there unite to form the so-called 

 anatomical ganglion of the spinal nerves. (Fig. o.) 



Each cell is spindle-shaped and gives off two processes : 



