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



smooth single-cell layer, approaches the differentiated cells in the central portion. 

 On the left, too, similar typical ependyma is shown. In the central area, which 

 has been repeatedly described, the elongated nuclei, the strands of protoplasm, and 

 the ragged, irregular intraventricular surface are well presented. The photomicro- 

 graph has been reproduced to show the relation of this differentiated area to the 

 various blood-channels in the supporting mesenchyme. Apparently the whole ven- 

 tricular roof is, at this stage, a site for an extensive capillary plexus; from both 

 sides, as shown in figure 35, vessels (one of great caliber) approach the central area 

 of differentiation. Directly beneath this area smaller capillary channels can be 

 made out, from which, apparently, a slight extravasation of red blood-cells has 

 occurred. Here, as in the greater part of the basilar pericerebral region, extravasa- 

 tion of the blood-cells is very frequent. This phenomenon has already been pointed 

 out by Mall 136 '. 



The large extent and the great differentiation of this peculiar area in the roof 

 of the fourth ventricle are well shown in figures 36 and 37, taken from a transverse 

 section of a pig embryo of 18 mm. In the photomicrograph of higher magnifica- 

 tion the two sharp tongues of typical ependyma are quite striking. Their abrupt 

 termination in the wide, differentiated area has nowhere been more convincingly 

 shown. The resemblance of these lining cells in the central area to the mesenchymal 

 elements adjoining is here also seen. The most interesting of all the phenomena 

 exhibited in this reproduction, however, is the attachment, apparently by precipi- 

 tation, of the coagulated albumen of the cerebro-spinal fluid. This coagulation, in 

 this specimen, delimits the differentiated area in the roof of the fourth ventricle. 

 The phenomenon is seemingly only an amplification of a similar attachment of 

 small fragments of the albuminous precipitate shown in other figures. 



Beyond the stage of 18 mm., which may be termed the maximal stage, the 

 differentiated area in the roof of the fourth ventricle undergoes a regression. This 

 is apparently due to the morphological alterations in this rhombic roof. The cho- 

 rioid plexuses in embryos over 18 mm. long deeply invaginate the fourth ven- 

 tricle, possibly drawing some of the true roof with them, but surely encroaching 

 upon the mid-line with their lateral tuftings. This growth tends to decrease the 

 available extent of the differentiated area, but an even more potent factor is the 

 rapid development of the cerebellum. The caudal growth of the cerebellar lip soon 

 largely occupies or replaces the superior half of the roof. These two factors, the 

 cerebellar growth and the enlargement of the chorioid plexuses, render the per- 

 sistence of the differentiated area impossible, so that a regression or disappearance 

 is to be expected. 



With these considerations before us, the study of sectioned pig embryos of a 

 greater length than 18 mm. becomes important. The process of disappearance, 

 however, does not occur at once. Thus, in an embryo pig of 19 mm. (figs. 42 and 43) 

 the differentiated area is as large and as characteristic as in the stage of 18 mm. 

 This same appearance and maintenance of size may be observed through the next 

 several millimeters' growth, but in pig embryos of 23 mm. the chorioid plexus has 



