PLATE 16 



EXPLANATION OF FIGURES 



81 to 87 Represent true transverse sections through what has been termed 

 in the text, the typical embryonic spinal cord, from a number of different verte- 

 brates, all of which have developed a tubular nervous system after the neural 

 fold method. They were drawn with the aid of an Edinger-Leitz drawing appara- 

 tus and reduced one half in reproduction. 



81 From a transverse section through the anterior portion of the spinal cord 

 of a 10 mm. Squalus embryo (Professor Scammon's series No. 16). This so- 

 called typical embryonic spinal cord is decidedly compressed. An earlier stage 

 possessed an elliptical cord with its greatest diameter from side to side. The 

 floor plate is slightly thicker than the roof plate. Each contains a single layer 

 of nuclei. The ventral portion of the cleft-like central canal is expanded into 

 a cavity, which persists as the permanent central canal. A well-formed spinal 

 ganglion is seen to the left. X 125. 



82 Transverse section of the spinal cord of a 19 mm. Squalus embryo (from 

 Professor Scammon's series No. 2). Note that the dorsal closure of the lateral 

 plates, due to fiber and cell proliferation, is the same as was figured for Petromy- 

 zon. They meet in a seam, leaving dorsal and ventral cavities, of which only 

 the ventral one persists. As in Cyclostomes this method of closure would tend 

 to throw a large part of the embryonic cerebro-spinal fluid into the brain cavities. 

 X 70. 



H'6 and 84 Transverse section through the anterior portion of the spinal 

 cord of an Amblystoma and a turtle embryo. The former (taken from Professor 

 Johnston's series No. 50) is a rather early representative of the so-called typi- 

 cal embryonic stage; while the latter is a rather late representative of this stage. 

 Both cords may be said to be compressed (elliptical, having its greatest diameter 

 dorso-ventral), but only slightly so, when compared with birds and mammals 

 (figs. 85 and 86). As a result, granting an equal proliferation of fibers and cells 

 in the lateral plates, it would be expected that the adult cord in Amblystoma and 

 the turtle would be more depressed, which is found to be the case. X 70 and 125. 



85 and 86 From anterior transverse sections of the spinal cord of a 93 hour 

 chick and a 5 mm. pig. Both are good illustrations of the so-called typical em- 

 bryonic stage, the pig being in a slightly more embryonic state. In these we have 

 the most compressed of all embryonic cords examined, while the adults cord are 

 nearly cylindrical. X 125. 



87 Transverse section through the caudal end of the same spinal cord shown 

 in figure 86. Observe spherical appearance which is indicative of an earlier phase 

 in its development. X 125. 



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