PLATK 11 



KXl'LANATION OK FIGUHI> 



51) ;uul t>() Two I laiisvciisc sections only 290 nii(;rons ;i[)urt through the ex- 

 treme posterior end of the spinal cord of a 70 mm. Polistotrema embryo. In 

 the more anterior section (fig. 59) the spinal cord is depressed, contains develop- 

 ing nervous elements, and the notochord is large proportionately. In the pos- 

 terior section (fig. (50) the diameter of the notochord is much reduced and only 

 supporting elements appear in the spinal cord. As a result no flattening of the 

 spinal cord has taken place in this region. Compare with figures 21 and 22, which 

 are similar sections through an adult Polistotrema. X 70. 



61 and 62 Taken from two transverse sections 480 microns apart, through 

 the medulla oblongata of an adult Polistotrema. In both sections no nervous 

 structures have appealed that were not present in the spinal cord. Note as you 

 pass caudad (figs. 62 to 61) that the medulla becomes flattened ventrally and dor- 

 sally in direct proportion to the increase in size of the notochord. This rela- 

 tionship can be shown fully as marked in more anterior sections, and in sections 

 taken from a similar region of larval Petromyzon. X 25. 



ABBREVIATIONS 



Aud.V., auditory vesicle or otocyst M.V.C., median ventral cartilaginous 



B.V., blood vessel bar 



C.C., central canal Myo., myotomes 



E.N., undifferentiated embryonic nu- N.A., membranous neural arch 



clei N.C., nerve cell 



Ep., ependyma P.M., pia mater or meningeal mem- 

 Ep.N., layer of ependymal nuclei brane of the younger stages 



L.S., lateral veno-lymphatic sinus or P.P., parachordal plate 



anlage of the same Sp.G., spinal ganglion 



M.D.C., median dorsal cartilaginous W.M., white matter 



bar X.G. in figure 62 should be Sp.G. 



M.F., Miillerian or giant fiber X.N., vagus nerve 

 M.L., mantle layer 



PLATE 12 



EXPLANATION OF FIGURES 



63, represents a diagrammatic reconstruction of the fourth ventricle from an 

 adult Polistotrema series, and the planes from which the transverse sections were 

 drawn for figures 64 to 66 are indicated by lines bearing those figures. Observe 

 how the large fourth ventricle of the embryo has been reduced to a small central 

 canal, having a posterior dilation (P4F), and how the anterior end breaks up 

 into two or more small longitudinal canals that soon terminate in the sinus 

 mesocoelicus. 



64 and 65, represent transverse sections, taken at different levels of the fourth 

 ventricle of Polistotrema. These sections were drawn with the aid of an Edin- 

 ger-Leitz drawing apparatus and were reduced one half. 



64 From a transverse section through the caudal portion of the mesencephalic 

 lobes (cerebellum of Miss Worthington). Exact plane indicated by line 64 in 

 figure 63. The section passes through the posterior mesoccele or cerebellar ven- 

 tricle {M" .) and the sinus mesocoelicus (anterior dilation of the fourth ventricle 

 of Miss Worthington), a short distance behind the branching off of the posterior 

 mesocoele. The cavity contains a fibrillar feltwork, wliich is in part coagulated 

 cerebro-spinal fluid and in part ependymal cilia. The ependyma surrounding 

 the fourth ventricle is rich in blood vessels, which derives its arterial sui)ply from 

 {Continued on page 68) 



