OF ARTS AND SCIENCES. 101 



absorption of the epithelial wall, formed by the fusion of its two sides, 

 a fissure between the two halves of the spinal cord becomes formed." 

 This is the postei'ior fissure. During its formation the white matter 

 of the dorsal columns becomes prolonged so as to line its walls. 



Material. 



The present investigations were made exclusively on embryos of 

 the pig. 



When sufficiently small these were placed for a few hours in picro- 

 sulphuric acid,* and then hardened in alcohol. In the latter process 

 stronger grades were gradually substituted for the weaker, beginning 

 with 70 per cent. 



To render embryos in which ossification had begun suitable for sec- 

 tioning, they were either placed in toto in picro-nitric acid long enough 

 to remove the salts of lime and then hardened, as in the case of younger 

 embryos, or the desired portions were cut from the embryos and treated 

 iu the same manner. 



Most of the specimens thus prepared were stained in a neutral 

 aqueous solution of carmine before cutting ; some, however, were 

 stained in picro-carmine ; the results were less satisfactory with the 

 latter method. 



The measurements were taken from the tip of the nose to the root 

 of the tail following the curvature of the back.f 



Observations. 



In embryos 43 mm. long cross sections through the posterior part 

 of the lumbar enlargement show that the dorsal wall of the central 

 canal is still composed exclusively of epithelial cells. 



* Kleinenberg's preparation, viz. : Sat. aq. sol. picric acid, 100 parts ; cone, 

 sulphuric acid, 2 parts ; diluted with three times its volume of distilled water. 



t The following table will enable the reader more easily to compare the stages 

 I have employed with those used by other observers, who have usually given 

 the length as measured from the mid-brain to the most distant point of the em- 

 bryo in its normal curved position. 



Measured along the curvature. Measured in a straight line. 



43 mm 23 mm. 



54 " 29 " 



65 " 35 



100 " 65 



125 " 78 



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u 



