EXPLANATION OF PLATES. 113 



PLATE VIII. 



Flo. 24. Photomicrograph of a transverse section of a pig embryo of 8 mm. Fixed in modified Bouin's solution over 

 night, dehydrated by 2 and 4 per cent grades of alcohol, embedded in xylol-paraffin. Enlargement, 30 

 diameters. 



FIG. 25. Photomicrograph, retouched, of the blocked area in fig. 24. The character of the cells (epc) composing the 

 roof of the fourth ventricle (4ve) is shown in this reproduction. Enlargement, 165 diameters. 



FIG. 26. Photomicrograph of a sagittal section from a pig embryo of 11 mm. Fixed in modified Bouin's solution over 

 night, dehydrated by 2 and 4 per cent grades of alcohol, embedded in xylol-paraffin. Enlargement, 11 

 diameters. 



FIG. 27. Photomicrograph of the blocked area in fig. 26. The area membranacea superior (ams) in the roof of the 

 fourth ventricle is shown sharply delimited from the two processes of typical ependyma (epe). Enlarge- 

 ment, 67 diameters. 



Fio. 28. Photomicrograph of a more lateral section of the pig embryo of 11 mm. given in fig. 26. Enlargement, 11 

 diameters. 



FIG. 29. Photomicrograph, under higher magnification, of the blocked area in fig. 28. The lateral border of the area 

 membranacea superior (ams) of the roof of the fourth ventricle is given. Enlargement, 50 diameters. 



Fio. 30. Photomicrograph of a sagittal section from a pig embryo of 13 mm. Fixed in modified Bouin's solution, 

 dehydrated by 2 and 4 per cent grades of alcohol, and embedded in xylol-paraffin. Enlargement, 8 

 diameters. 



Fio. 31. Photomicrograph, under higher magnification, of the squared area in fig. 30. The reproduction comprises a 

 sagittal section of the area membranacea superior (ams) of the roof of the fourth ventricle. Enlargement, 

 67 diameters. 



FIG. 32. Photomicrograph of a sagittal section of a pig embryo of 14 mm. Fixed in modified Bouin's solution, dehy- 

 drated by 2 and 4 per cent grades, and embedded in xylol-paraffin. Enlargement, 11 diameters. 



FIG. 33. Photomicrograph of the blocked area in fig. 32 under higher magnification. The area membranacea superior 

 (ams) in the roof of the fourth ventricle is reproduced. Enlargement, 75 diameters. 



PLATE IX. 



FIG. 34. Photomicrograph of a transverse section of a pig embryo of 18 mm. Fixed in Carney's fluid (6:3: 1), dehy- 

 drated by 2 and 4 per cent changes of alcohol, and embedded in xylol-paraffin . Enlargement, 13 diameters. 



FIG. 35. Photomicrograph, under higher magnification, of the blocked area in fig. 34. The area membranacea superior 

 (amg) is here given, flanked on either side by typical ependyma (epe). Enlargement, 170 diameters. 



FIG. 36. Photomicrograph of a transverse section of a pig embryo of 18 mm. Fixed in modified Bouin's fluid, dehy- 

 drated by 2 and 4 per cent changes, and embedded in xylol-paraffin. Enlargement, 13 diameters. 



FIG. 37. Photomicrograph of rectangular area outlined in fig. 36. The extent of the area membranacea superior (ams), 

 with its adherent coagulum of albuminous material, is well differentiated from the adjacent typical ven- 

 tricular ependyma (epe). Enlargement, 100 diameters. 



Fio. 38. Photomicrograph of a transverse section of a pig embryo of 19 mm. Fixed in modified Bouin's solution, 

 dehydrated by 2 and 4 per cent grades, and embedded in xylol-paraffin. Enlargement, 13 diameters. 



Fio. 39. Photomicrograph, under higher power, of the rectangular area in fig. 38. A small break in the integrity of 

 the lining ependyma of the roof of the fourth ventricle, representing the irregular boundary of the area 

 membranacea superior (ams), is given. Enlargement, 290 diameters. 



Fio. 40. Photomicrograph of a transverse section of a human embryo of 4 mm. (No. 836 of collection of Carnegie 

 Institution of Washington). Enlargement, 33 diameters. 



FIG. 41. Photomicrograph, retouched, of the blocked area in fig. 40. The epithelial-like cells (epc) composing the 

 roof of the fourth ventricle (4ve) are here shown separated from the denser nervous tissue. Enlargement, 

 100 diameters. 



PLATE X. 



Fio. 42. Photomicrograph of transverse section of pig embryo of 19 mm. Fixed over night in modified Bouin's 

 solution, dehydrated by 2 and 4 per cent changes of alcohol, and embedded in xylol-paraffin. Enlarge- 

 ment, 13 diameters. 



FIG. 43. Photomicrograph of squared area in figure 42, under higher magnification. The area membranacea superior 

 (ams) with the attached coagulum of albumen is reproduced. Enlargement, 115 diameters. 



FIG. 44. Photomicrograph of sagittal section of pig embryo of 23 mm. Fixed in modified Bouin's fluid, dehydrated by 

 2 and 4 per cent changes, and embedded in xylol-paraffin. Enlargement, 5 diameters. 



Fio. 45. Photomicrograph, under higher magnification, of squared area in fig. 44. The area membranacea superior 

 (ams) is here shown, delimited by the cells of the chorioid plexus (chp) on one side and by the further 

 ependymal prolongation (epe) of the cerebellar lip. Enlargement, 88 diameters. 



Fio. 46. Photomicrograph of sagittal section of pig embryo of 32 mm. Fixed in modified Bouin's solution, dehydrated 

 by 2 and 4 per cent changes, and embedded in xylol-paraffin. Certain portions of the dura mater (dmc) 

 are indicated. Enlargement, 5 diameters. 



FIG. 47. Photomicrograph of blocked area in fig. 46, under higher magnification. The small remaining area mem- 

 branacea superior (ams) is quite surrounded by encroaching ependyma in the chorioidal folds. En- 

 largement, 88 diameters. 



