UNDESCRIBED STRUCTURES IN ROOF OF THE FOURTH VENTRICLE. 45 



of the medulla, but the area is larger and the histological character more nearly 

 approaches the permanent feature of the tissue. The nuclei in this zone are paler 

 than those of the adjoining ependymal elements and contain less chromatin. The 

 cytoplasm is not scanty, nor is it very abundant in amount. The area is also 

 characterized by the occurrence of the cells in a layer, two or three cells in thickness. 



In view of the very slow differentiation of the area membranacea inferior in the 

 growth of the embryo from 15 to 18 mm., the enormous enlargement of the region 

 within the next few millimeters' growth is very astonishing. This period, as has 

 been pointed out, is a critical one in the extension of the embryonic cerebro-spinal 

 fluid from a ventricular to a periaxial relationship. Apparently, in the course of 

 the embryo's growth during these next few millimeters the whole inferior roof of 

 the ventricle undergoes a transformation and enlargement, so that the differentiated 

 area membranacea comes to occupy practically the whole inferior half of the roof. 

 This portion of the roof, persisting, enlarging, and suffering no extension of nervous 

 tissue upon it, becomes the tela chorioidea inferior. 



The rapid differentiation of the whole inferior half of the roof of the fourth 

 ventricle is a very interesting process. Apparently the typical ependymal ele- 

 ments, visible on both sides of the membranous area in figure 73, undergo a very 

 rapid alteration, so that in the course of a few millimeters' growth the cubical lining 

 of the ventricle is replaced by a low-type cell, with round or oval nuclei, staining 

 much less densely than do the ependymal elements. The whole area membranacea 

 rapidly becomes a membrane in the true sense of the word; it is a continuous, intact 

 layer of cells, generally only one cell in thickness, closing in the fourth ventricle 

 from the chorioid plexus above and the bulbar lips on the sides. 



The general characteristics of this transformation are seen in figures 74 and 75. 

 These photomicrographs are taken from a sagittal section of a pig embryo of 23 mm. 

 On one side of the sharply delimited membrane shown in figure 75 is a tongue of 

 nervous tissue of the medulla; on the other is the differentiated ependyma of the 

 chorioid plexus; between these two structures stretches uninterruptedly the area 

 membranacea inferior. The flattened cells of the membrane, with their oval nuclei 

 and almost continuous cytoplasm, effectually close the whole ventricle. The pho- 

 tomicrograph also shows an interesting characteristic of this membranous area 

 which is universally present in the larger forms; this is the relatively unsupported 

 character of the membrane. The highly vascular mesenchyme posterior to the area 

 has gradually developed, during growth, larger and larger interstices between the 

 cytoplasmic processes. The phenomenon is not due to shrinkage, but is intimately 

 connected with the formation of the future cisterna cerebello-medullaris. This 

 phase of the mesenchymal differentiation will be more fully considered in an appro- 

 priate section of this paper. It will suffice here merely to record the lack of support 

 of the membrane. 



Another phenomenon of importance in the cerebro-spinal fluid relationships of 

 this stage is shown in figure 75. In the mesenchymal spaces directly beneath the 

 membranous area there is a large amount of albuminous coagulum. This phenome- 



