T^iE PERIPHEEAL NERVOUS SYSTEM IN THE HUMAN 

 EMBRYO AT THE END OF THE FIRST MONTH (10 mm.). 



GEORGE L. STREETER, M.D., 



Professoi- of Anatomy, University of Michigan, Ann Arbor. 



(With 3 Plates and 1 Text Figure.) 



The period of development under consideration represents the com- 

 pletion of what we may call the primary stage in the growth of the 

 nervous system. The primary neurones forming the peripheral nerves 

 are by the end of the first month well laid down; all their chief periph- 

 eral branches and plexuses are indicated, and centrally the nerve roots 

 can be traced to their distribution in the brain and spinal cord, where 

 the nuclei of the motor roots can be outlined and the sensory roots can 

 be recognized as definite bundles extending up and down in the wall 

 of the neural tube. The higher neurone systems, however, are still 

 in a most rudimentary state, and in sections through the brain and cord 

 at this time we see only the primary apparatus differentiated. Such 

 co-ordinating centers as the pons, olive and cerebellum are still undevel- 

 oped, and the forebrain is not much more than an undifferentiated 

 thin-walled tube. It can thus be seen that the period with which we are 

 dealing represents a definite stage in the growth of the nervous system, 

 the stage of the primary brain, and a stage which is of particular impor- 

 tance for a proper conception of the embryology of this system. 



A general view of the nervous system as it exists in embryos at 

 the end of the first month is represented on Plate I. It can bo seen 

 that the reconstruction shown there corresponds in age almost exactly 

 with the well known His reconstruction of his embryo KO of 10.3 mm. 

 Nl. That investigator early recognized the significance of the stage 

 of the primary brain. However, since His, '88, published his mono- 

 graph and description of the embryo KO there have been introduced 

 many improvements in the methods of work, and before all others 

 should be mentioned the Born wax plate procedure. It is with such 

 The American Jodenal of Anatomy.— Vol. VIII, No. 3. 



