114 DEVELOPMENT OF CEREBRO-SPINAL SPACES IN PIG AND IN MAN. 



PLATE X Continued. 



FIG. 48. Photomicrograph of transverse section of human embryo of 7 mm. (No. 617 of the collection of the Carnegie 



Institution of Washington). Enlargement, 10 diameters. 

 FIG. 49. Photomicrograph of squared area in fig. 48, under higher magnification. The epithelial-like cells (epc) 



composing the roof of the fourth ventricle at this stage are well shown. Enlargement, 100 diameters. 

 Fio. 50. Photomicrograph of transverse section of human embryo of 7 mm. (No. 617 in the Carnegie Institution of 



Washington). Enlargement, 10 diameters. 

 FIG. 51. Photomicrograph of blocked area in fig. 50. The marked invagination of the roof of the fourth ventricle 



(4ve) with the lining of epithelial-like cells (epc) is given. Enlargement, 33 diameters. 

 FIG. 52. Photomicrograph of transverse section of human embryo of 9 mm. (No. 721 in the collection of the Carnegie 



Institution of Washington). Enlargement, 10 diameters. 

 FIG. 53. Photomicrograph of squared area outlined in fig. 52. The pale, large cells (epc) comprising the roof of the 



fourth ventricle characterize the reproduction. Enlargement, 50 diameters. 

 Fio. 54. Photomicrograph of sagittal section of human embryo of 11 mm. (No. 544 in the collection of the Carnegie 



Institution of Washington). Enlargement, 6 diameters. 

 FIG. 55. Photomicrograph of blocked area in fig. 54. The apparent break in the continuity of the roof of the fourth 



ventricle with exudation of the ventricular albumen into the mesenchyme is brought out. Enlargement, 



50 diameters. 



PLATE XI. 



Fio. 56. Photomicrograph of sagittal section of human embryo of 14 mm. measured on the slide (No. 144 of the col- 

 lection of the Carnegie Institution of Washington). Enlargement, 8 diameters. 

 FIG. 57. Photomicrograph, under higher magnification, of blocked area in fig. 56. The greater part of the ventricular 



wall shown is composed of the area membranacea superior (ams), bounded below by typical ventricular 



ependyma (epe). Enlargement, 67 diameters. 

 FIG. 58. Photomicrograph of sagittal section of human embryo of 17 mm. (No. [576 of the collection of the Carnegie 



Institution of Washington) . Enlargement, 10 diameters. 

 FIG. 59. Photomicrograph of rectangular area in fig. 58, showing the area membranacea superior (ams) of the roof of 



the fourth ventricle. Enlargement, 50 diameters. 

 FIG. 60. Photomicrograph of sagittal section of human embryo of 17 mm. (No. 576 of the collection of the Carnegie 



Institution of Washington). Enlargement, 7 diameters. 

 FIG. 61. Photomicrograph of the blocked area in fig. 60 under higher magnification. The aggregation of epithelial-like 



cells (epc) on the lateral border of the area membranacea superior is here portrayed. Enlargement, 



67 diameters. 

 FIG. 62. Photomicrograph of transverse section of human embryo of 18 mm. (No. 409 of the collection of the Carnegie 



Institution of Washington) . Enlargement, 7 diameters. 

 FIG. 63. Photomicrograph, under higher power, of squared field in fig. 62. The peculiar inversion of the roof of the 



fourth ventricle (4ve) indicated in fig. 62, has resulted in a marked dislocation of the area membranacea 



superior (ams), shown in this figure. Enlargement, 75 diameters. 



PLATE XII. 



FIG. 64. Photomicrograph, retouched, of a transverse section of a human embryo of 21 mm. (No. 460 of the collection 

 of the Carnegie Institution of Washington). The field taken consists of a portion of the fourth ventricle 

 with the lining of typical ependyma (epe) on either side. The area membranacea superior (ams) is 

 shown between the two lips of ependyma. Enlargement, 33 diameters. 



FIG. 65. Photomicrograph, retouched, of a similar section to that given in fig. 64, but taken from a more anterior 

 plane from the same embryo. The field shown is analogous in every way to that in the preceding figure. 



FIG. 66. Photomicrograph of a transverse section of an embryo chick of 121 hours' incubation. Fixed in Bouin's solu- 

 tion. Enlargement, 15 diameters. 



FIG. 67. Retouched photomicrograph, under higher magnification, of the blocked area in fig. 66. The area mem- 

 branacea superior (ams) is here given, delimited sharply from the lips of ependyma (fpe) which line the 

 roof of the fourth ventricle. Enlargement, 133 diameters. 



FIG. 68. Photomicrograph of a more caudal section from the same embryo as portrayed in fig. 66. Enlargement, 15 

 diameters. 



FIG. 69. Retouched photomicrograph, under higher magnification, of the blocked area in fig. 68. The area mem- 

 branacea superior (ams) is shown at the point of its greatest transverse diameter. Enlargement, 88 

 diameters. 



FIG. 70. Photomicrograph of a sagittal section of a pig embryo of 15 mm. Fixed in modified Bourn's solution, dehy- 

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



FIG. 71. Photomicrograph, under higher magnification, of blocked area in fig. 70. The earliest evidence of the area 

 membranacea inferior (ami) in the roof of the fourth ventricle is here shown. Enlargement, 125 

 diameters. 



