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



those lining other parts of the ventricular cavities. These cells are quite unlike 

 the deeply staining ependymal cells, which can be so readily identified as the lining 

 cells in older embryos. In this younger stage of 8 mm., the entire ventricular roof 

 is composed of several layers of cells with round or somewhat oval nuclei and fairly 

 abundant cytoplasm. The cell-boundaries are not well denned. The nuclei are 

 not deeply tinged with hematoxylin. The chromatin material is sparse and irregu- 

 larly distributed. Nucleoli are prominent. The cytoplasmic border lining the ven- 

 tricular cavity is rough and ragged at times, often blending with the coagulated 

 albumen of the cerebro-spinal fluid. Altogether, these lining cells bear a much 

 greater resemblance to the epithelial cells than to the ependymal. 



These characteristics of the lining cells of the roof of the fourth ventricle are 

 shown in figures 24 and 25, from a pig embryo of 8 mm. The close association of 

 the roof cells to the surface epithelium is easily made out hi figure 25, as well as the 

 general character of the lining cells. 



At the stages of 8 mm. and under, in the pig embryo, the roof of the fourth 

 ventricle is relatively quite large. In its whole extent it is formed of the peculiar 

 lining cells described above. With the growth of the embryonic nervous system, 

 the roof of the fourth ventricle is subjected to alterations in form and position; to 

 some extent these changes influence the cells which line the cavity in the early stages. 



In pig embryos between 8 and 12 mm. in length the roof of the fourth ventricle 

 undergoes a change. The ependyma, which from comparison with later stages is 

 regarded as typical, begins to encroach upon the epithelial-like cells which are so 

 numerous in the 8 mm. stage (fig. 25). The area occupied by these cells diminishes, 

 not only relatively but absolutely. It becomes smaller and the cells gradually 

 change their character. These changes are shown in figures 26 and 27, from a pig 

 embryo of 11 mm. Figure 26 gives the location, in a sagittal section near the mid- 

 line of the area in figure 27, taken at a higher magnification. 



In figure 27 the densely staining lips of ependymal and nerve cells are seen 

 approaching each other. For a considerable space in the central portion of the 

 photograph there is an area similar to that shown in figure 33. But considered in 

 connection with figure 25 this area represents the epithelial-like cells of the roof of 

 the fourth ventricle. This relationship is more clearly shown in figures 28 and 29, 

 taken in a more lateral plane from the same embryo (11 mm.). Examination, how- 

 ever, of the area in figure 29 shows the epithelial-like cells again apparent in the roof 

 of the fourth ventricle. 



The process of transformation, then, as shown in these photographs from an 

 embryo pig of 11 mm., concerns a gradual encroachment upon the area of epithelial- 

 like cells by the more densely staining and more closely packed ependymal cells. 

 Gradually the epithelial-like cells in the central portion of the area lose their former 

 character (fig. 27), while around the periphery, especially on the lateral sides, the 

 epithelial-like appearance persists (fig. 29). 



On the lateral side of this area, just as the typical ependymal lining is about to 

 become isolated (fig. 29), the epithelial-like lining cells form a several-celled layer. 



