57 



like the central canal, with ciliated epithelium. This secondary canal 

 rapidly becomes larger, and shifts its position so that it lies well down 

 in the left lateral tract, midway between the margin of the cord and 

 the central canal. In the same section, two smaller secondary canals 

 appear, one lying in the roof- 

 plate almost in the mid-line. 

 The central canal is here much 

 narrower dorso-ventrally than in 

 a normal [cord, and is pushed 

 over to the right. In a few 



fc). ; V^ : ¥m W0-0M c 



Fig. 1. Cross section of nearly nor- 

 mal spinal cord before the appearance 

 of secondary canals, c, c. central canal. 



All outlines were drawn with the 

 camera, and with the same magnification 

 (about 80 diam.). 



more sections the large canal trails off, leaving no trace behind, and 

 the small one disappears at almost exactly the same level. When it 

 has disappeared, a new one begins. The next section shows four dark 

 circular patches, and three of these soon show an elliptical lumen. 

 They are very short, disappearing in a few sections, and must be 

 nearly spherical. The central canal is rapidly growing shorter, being 

 cut off from the dorsal side, and has acquired an elliptical outline. 



For a few sections the cord is without secondary canals, and then 

 a new one appears in the position of the first, and like the first 

 changes its position and 

 lies in the left lateral tract. 

 It is the largest and most 

 persistent of the secondary 

 canals, almost equalling in 

 size the central canal. 

 After this canal has dis- 

 appeared, a small one 

 appears on the dorsal mid- 

 line, and at the same time 

 another small one is found 

 in the dorsal horn of the 

 right side, which swells a 



Fig. 2. C. S. Cord with two secondary canals; the one on the right approaches 

 more nearly to the central canal than any other secondary canal in the cord. Central 

 canal small, c. c. central canal. S. C. secondary canal. 

 Letters have the same reference in all the figures. 



