SPINAL CORD AND MEDULLA OF CYCLOSTOMES 11 



Anatomy of which Prof. C. M. Jackson is director, were espe- 

 cially valuable. 



It is a great pleasure to the writer to have this opportunity 

 of expressing his obligations to Prof. J. B. Johnston for his 

 valuable suggestions and friendly criticism of this work. 



A model of the caudal end of the spinal cord from a region a 

 little in front of the caudal hearts to its extreme tip was pre- 

 pared in four sections from the 20 cm. Polistotrema series. 

 Also a model in two pieces was prepared of the cavity of the so- 

 called first roof plate expansion and the enlarged central canal 

 of the above mentioned series of Polistotrema. These models 

 were constructed out of blotting paper after a modification of 

 the Born method. Tracings of each section were made on 

 ordinary writing paper with the aid of an Edinger-Leitz draw- 

 ing apparatus, using a magnification of 100 diameters, and 

 afterward each tracing was carefully checked up for accuracy 

 with a higher magnification. At the very outset two base or 

 projection lines wore drawn on the first tracing, one following 

 the median longitudinal plane, and the other a horizontal line 

 drawn at right angles to the first line, passing along the ventral 

 border of the notochord. These projection lines were added by 

 pencil to all of the succeeding tracings after the following manner: 

 The second tracing was carefully fitted over the first, after i)lac- 

 ing both over a plate glass covered box, containing an electric 

 light reflected upward, and in like manner these lines were added 

 to the third tracing from the second, and so on to the end of the 

 series. Then with the aid of carbon paper these tracings and 

 projection lines were transferred to sheets of blotting paper, 

 having a definite thickness, previously determined after the 

 following method. When thoroughly cooled after an immersion 

 in melted paraffin, the blotting paper selected should have a 

 thickness, equal to the thickness of the section multiplied by 

 the magnification used, which in this case totalled 1.5 mm. The 

 sheets of blotting paper containing the transposed tracings were 

 then inmiersed in melted paraffin, drained and cooled. After 

 which the outlines of the tracings were cut out with a sharp 

 scalpel, and the sections were built up in regular order. In 



