286 George L. Streeter 



aid and by means of the increased supply of available embryos that it 

 was possible by Lewis, '02, to elicit further detail and obtain greater 

 completeness in our conception of the formation of the brachial plexus 

 and the development of the nerves of the arm, and Bardeen, '07, the 

 nerves of the leg, and myself the nerves of the occipital region (Streeter, 

 '04). With this in mind it seemed desirable to go over the same ground 

 covered by His in his reconstruction of the embryo KO with the purpose 

 of bringing out the same structures with more accurate detail and in 

 more plastic form, and at the same time to incorporate the results of 

 the work of the investigators just mentioned. 



Some of the features brought out by this study with regard to the 

 cranial nerves were reported at the Chicago meeting of the Association 

 of American Anatomists (Streeter, '08). It is the purpose of the 

 present paper to include the whole peripheral nervous system of the same 

 embr}'0. 



Material and Methods. The embryo on which this study is based 

 constitutes No. 3 of Professor Ruber's Collection, and was kindly loaned 

 by him for this purpose. After fixation in alcohol the specimen meas- 

 ured 10 mm. greatest length. Its estimated age is 31 days. Unfortu- 

 nately there was no clinical history obtainable. The embryo is cut in 

 a series of 5 micra sagittal sections, and the tissues are in an excellent 

 state of preservation. The description and drawings are based in part 

 on wax plate models and in part on profile reconstructions made in the 

 usual way by superimposing transparent papers. 



Brain" and Spinal Cord. 



The general form of the brain and spinal cord and their relation to 

 the body outlines is shown in Plate I. The spinal cord is largest in 

 the cervical region, and from there slowly tapers down, being somewhat 

 larger again in the lumbar region, and finally abruptly tapers off at 

 the coccyx. In transverse section the cord possesses a trapezoid out- 

 line with rounded corners, the width of the ventral half being a little 

 greater than that of the dorsal half. A large ventricle or central canal 

 extends throughout its whole length. By the lanceolate border of this 

 canal the cord is divided on each side of the median line into an alar and 

 basal plate. 



Toward the head the spinal cord enlarges and gradually merges into 

 the rhombencephalon. In this transition the basal plates become thicker, 



