M WILLIAM F. ALLEN 



Inter sta^e (fig. 57) a nieciiaii dorsal cartilage has been formed. 

 The inuoh stronger membranous neural arches are firmly attached 

 above to this dorsal cartilage and l)elow to the notocliord. The 

 soft plastic spinal cord is thus closely confined by the dorsal 

 cartilage above and less closely by the neural arches laterally. 

 The notochord beneath it has gi'own very rapidly and its enor- 

 mous increase in size has brought about a decided flattening 

 (depression) of the spinal cord and indentation of its ventral 

 surface. 



It is equally clear in Petromyzon also that the growing noto- 

 chord is to be looked upon as a direct cause for bringing about 

 the flattening of the spinal cord. The external conditions sur- 

 rounding the spinal cord are shown in figure 58 to be equally 

 favorable for assisting the notochord in this process, with the 

 possible exception that the membranous neural arch is attached 

 abo\'e to a membranous neural spine instead of a cartilage, which, 

 however, may be compensated for by an increased dorsal growth 

 of the myotomes. 



The hypothesis that the flattening of the spinal cord in Cyclo- 

 stomes is largely brought about by the upward growth of the 

 notochord after the manner set forth in the previous paragraph 

 is considerabl}^ strengthened by the fact that a certain relation- 

 ship exists between the size of the notochord and the amount 

 of flattening of the spinal cord. 



This is clear in the 20 cm. Polistotrema series from sections 

 of the posterior end of the spinal cord (figs. 19, 21, and 22) and 

 from the photographs of the model of the same region (figs. 7 

 to 9) ; the flattening (depression) of the spinal cord becoming- 

 less evident as the notochord decreases in size. More striking 

 is a similar relationship shown in the tail region of the 70 mm. 

 Polistotrema (figs. 59 and 60) for the reason that these two sec- 

 tions were only one-fourth of a millimeter apart. The above 

 relationship between the size of the notochord and the depres- 

 sion of the spinal cord can be demonstrated fully as conclu- 

 sively in the medulla region (see figs. 61 and 62). It should l)e 

 recorded for these two sections that their structure is the same 

 as that of the spinal cord and that they are less than one-half 



