UNDESCEIBED STRUCTURES IN ROOF OF THE FOURTH VENTRICLE. 33 



The nuclei are poor in chromatin material and the cytoplasm somewhat small in 

 amount. The inner cytoplasmic border lining the ventricle is in contrast, by its 

 ragged outline, with adjacent smoother ependyma on both sides. At this stage of 

 the pig embryo the characteristics of the epithelial-like cells are still to be made out, 

 but a gradual transformation is becoming evident. 



The metamorphosis becomes much more marked in the central portion of the 

 area, as shown in figures 26 and 27. In these figures the whole central area seems 

 to have lost some of its former character as an intact cell-layer. Closer examina- 

 tion, however, under higher power demonstrates that it still possesses an intact 

 surface as a lining for the ventricle. Delicate cytoplasmic strands stretch in a 

 continuous line across the whole area between the lips of denser typical ependyma. 

 The nuclei in this differentiated area are seemingly altered from their rounded form 

 and have elongated almost into spindles. The inner cytoplasmic border is charac- 

 teristically rough, with small amounts of coagulated albumen adhering to the pro- 

 cesses. The area, then, in its central portion, at the stage of 11 mm., has assumed 

 the character of the stage of 14 mm. (fig. 32) . On the periphery, however, the cells 

 still resemble those of smaller stages (8 mm.). 



From the pictures presented by the intermediate stages (figs. 27, 28, and 29) 

 the differentiation goes on very rapidly, so that in the pig embryo of 13 mm. there 

 is rarely any evidence of the epithelial-like cells. Figures 30 and 31 are photo- 

 micrographs of a sagittal section of an embryo pig of 13 mm.; here there are no 

 evidences of the epithelial-like cells. The whole area, pictured in figure 31 as 

 sharply delimited from the tongues of typical ependyma above and below, has 

 become well differentiated. The cell-character observed in figures 27 and 33 (elon- 

 gated nuclei and scanty strands of protoplasm) has become very obvious. The 

 ragged and roughened intraventricular border, the coagulated albumen, and the 

 abrupt transition from the neighboring typical ependyma are well shown in the 

 photomicrographs of this specimen. 



The differentiation of this area in the roof of the fourth ventricle of the pig 

 embryo proceeds at a very rapid rate, so that within the growth of a few millimeters 

 (from 8 to 13 or 14) a great histological change occurs. Figures 32 and 33, already 

 described, show the extent of this metamorphosis in a pig embryo of 14 mm. The 

 process, however, continues, modified possibly by the changing of the roof of the 

 fourth ventricle. For this roof structure is subjected to marked alteration in stages 

 of 14 mm. and upwards, both by the lateral development of the chorioid plexuses 

 and by the readjustment of the cervical and pontine flexures. Its maximal differ- 

 entiation may be said to appear at a stage of 18 mm.; this is maintained through 

 several millimeters, until undergoing final retrogression. 



This maximal change in the roof of the fourth ventricle is shown in figures 34, 

 35, 36, and 37. Several points of interest are brought out in these photomicro- 

 graphs. Figure 35 represents an enlargement of the rectangular area in figure 34, 

 taken from transverse sections of an embryo pig of 18 mm. The area is particularly 

 well shown in this figure, in which, from the right, the typical ependyma, in a fairly 



