EXPLANATION OF PLATES. 115 



PLATE XIII. 



FIG. 72. Photomicrograph of sagittal section of a pig embryo of 18 mm. Fixed in modified Bouin's solution, dehy- 

 drated by 2 and 4 per cent grades of alcohol, and embedded in xylol-paraffin. Enlargement, 1 1 diameters. 



FIG. 73. Photomicrograph, under higher power, of the rectangular area outlined in fig. 72. The enlarging area mem- 

 branacea inferior (ami) is shown in the midst of the typical lining ependyma of the roof. Enlargement, 

 100 diameters. 



FIG. 74. Photomicrograph of sagittal section of a pig embryo of 23 mm. Fixed in modified Bouin's solution, dehy- 

 drated by 2 and 4 per cent grades of alcohol, and embedded in xylol-paraffin. Enlargement, 6 diameters. 



FIG. 75. Photomicrograph of blocked area in fig. 74. The area membranacea inferior (ami) is, at this stage, quite 

 extensive, as shown in the reproduction ; the early stages in the development of the cisterna cerebello- 

 medullaris may also be seen. Enlargement, 75 diameters. 



FIG. 76. Photomicrograph of sagittal section of a pig embryo of 32 mm. Fixed in modified Bouin's solution, dehy- 

 drated by 2 and 4 per cent grades of alcohol, and embedded in xylol-paraffin. Enlargement, 7 diameters. 



FIG. 77. Photomicrograph, under higher magnification, of blocked area in fig. 76. The unsupported character of the 

 area membranacea inferior and the formation of the cisterna cerebello-medullaris is here reproduced. 

 Enlargement, 67 diameters. 



PLATE XIV. 



FIG. 78. Photomicrograph of a sagittal section of a pig embryo of 32 mm. Fixed in modified Bouin's solution, dehy- 

 drated by 2 and 4 per cent grades of alcohol, and embedded in xylol-paraffin. Enlargement, 7 diameters. 



FIG. 79. Photomicrograph of the blocked area in fig. 78, under higher magnification. The intact area membranacea 

 inferior (ami), unsupported by any mass of tissue, is shown separating the ventricular cavity from the 

 developing cisterna cerebello-medullaris. Enlargement, 67 diameters. 



FIG. 80. Photomicrograph of a sagittal section of a human embryo of 16 mm. (No. 406 in the collection of the Carnegie 

 Institution of Washington). Enlargement, 7 diameters. 



FIG. 81. Photomicrograph of the area outlined in fig. 80, but under higher magnification. An early stage in the differ- 

 entiation of the area membranacea inferior (ami) is given. Enlargement, 50 diameters. 



FIG. 82. Photomicrograph of a sagittal section of a human embryo of 17 mm. (No. 576 in the collection of the Carnegie 

 Institution of Washington). Enlargement, 6 diameters. 



FIG. 83. Photomicrograph, under higher power, of the area blocked in fig. 82. The chorioid plexuses of the fourth 

 ventricle he in the central portion of the field; above is the thick cell-layer on the lateral side of the area 

 membranacea superior (ams), while below the upper limit of the area membranacea inferior (ami) appears. 

 Enlargement, 67 diameters. 



Fia. 84. Photomicrograph of a transverse section of a human embryo of 18 mm. (No. 409 in the collection of the 

 Carnegie Institution of Washington) . Enlargement, 5 diameters. 



FIG. 85. Photomicrograph of the blocked area in fig. 84. The cellular character, and especially the clumping of cells, 

 of the area membranacea inferior (ami) is shown. Enlargement, 25 diameters. 



Fia. 86. Photomicrograph of a sagittal section of a human embryo of 19 mm. (No. 431 in the collection of the Carnegie 

 Institution of Washington). Enlargement, 5 diameters. 



FIG. 87. Photomicrograph of the blocked area outlined in fig. 86. The area membranacea inferior (ami) appears 

 separating the fourth ventricle from the developing cisterna cerebello-medullaris. Enlargement, 25 

 diameters. 



PLATE XV. 



FIG. 88. Photomicrograph from a sagittal section of a human embryo of 17 mm. (No. 576 of the collection of the 

 Carnegie Institution of Washington), representing an enlargement of the second blocked area in fig. 58. 

 The area membranacea inferior (ami) appears sharply delimited from the adjoining typical ependyma. 

 Enlargement, 67 diameters. 



FIG. 89. Photomicrograph of a sagittal section of a human embryo of 23 mm. (No. 453 of the collection of the Carnegie 

 Institution of Washington). Enlargement, 6 diameters. 



FIG. 90. Photomicrograph of the blocked area in fig. 89. The area membranacea superior (ams) appears in the stage 

 of closure, while the area membranacea inferior (ami) is becoming well differentiated from the typical 

 ependyma lining the other portions of the fourth ventricle. Enlargement, 26 diameters. 



FIG. 91. Photomicrograph of a sagittal section of a human embryo of 26 mm. (No. 1008 of the collection of the Carnegie 

 Institution of Washington). Enlargement, 4.5 diameters. 



Fro. 92. Photomicrograph, under higher magnification, of the blocked area in fig. 91. The area membranacea superior 

 has been almost completely closed by the dense ependyma of the superior half of the roof of the fourth 

 ventricle, while the inferior area (ami) has become a membrane lacking wholly the character of ependyma. 

 Enlargement, 23 diameters. 



FIG. 93. Photomicrograph of a sagittal section of a human embryo of 35 mm. (No. 199 of the collection of the Carnegie 

 Institution of Washington). Enlargement, 3 diameters. 



Fia. 94. Photomicrograph, under higher powers, of the blocked areas in fig. 93. The formation of the cisterna cere- 

 bello-medullaris is shown in relation to the ventricular roof. Enlargement, 23 diameters. 



FIG. 95. Drawing of cells of the chorioid plexus from the lateral ventricles of a fetal pig of 132 mm. The specimen was 

 fixed in absolute alcohol, and stained by Best's carmine stain for glycogen. The glycogen occurs in the 

 form of globules within the epithelial cells. Enlargement, 950 diameters. 



Fia. 96. Drawings of the cells of the chorioid plexus from the lateral ventricles of a fetal pig of 36 mm. The specimen 

 was fixed in absolute alcohol and stained by Best's carmine method. The glycogen appears in the epi- 

 thelial eclls in the form of basilar plaques. Enlargement, 950 diameters. 



