FLAri' 



EXPLANATION OF FIGURES 



4 I.aliMal vii'w of the left side of the second model of the Polistotreina spinal 

 curd, which should follow figure 1. Observe the continuation of the first roof 

 plate expansion noted in hgure 1 and two additional outcroppings of the roof 

 plate (/?. ^.r. 2 and 3). Between these outcroppings the spinal cord is perfectly 

 iionnal. Three motor and sensory roots are shown in the figure. X 50. 



.') Dorsal photograph of the same model as figure 4. The extent and posi- 

 tions of the three roof plate outcroppings previously mentioned in the description 

 of figure 4 are well portrayed here, as are also the sensory roots. X 50. 



6 Dor.sal view of a cast of the more caudal portion of the central canal and 

 cavity of the first roof plate expansion. This cast differs from the more anterior 

 cast in that there are no connections between the central canal and the cavity 

 of the roof expansion. Evidence from transverse sections favors the view that 

 a connection once existed, which has been cut ofY in later development. In the 

 two posterior outcroppings of the roof plate there is a connection between the 

 central canal and the cavity of the roof expansion. The light places in the photo- 

 graph are indicative of places in the roof expansion where it is solid and con- 

 tains no cavity. They are more numerous and decidedly larger than was shown 

 in the more anterior model (fig. 3). X 50. 



Between models 2 and 4 (figs. 4 and 7) comes model 3, the photograph of which 

 has not been included as a figure. It is about equal in length to models 2 and 4. 

 simply eoimecting the two, without presenting any peculiarities in roof plate 

 expansion, etc. 



ABBREVIATIONS 



M.R.. motor or v(Mitral spinal nerve R.Ex.{Z), third roof plate expansion 



root in the Polistotrema cord 



R.Ex., roof plate expansion S.H., sensory or dorsal spinal nerve 



R.Ex.i^Z), second roof plate expansion root 

 in the Polistotrema cord 



46 



