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



The pericerebral spread may be made out in nearly all replacements in embryos 

 of 14 mm., but in a few cases the injection has remained intramedullary in type. 

 In embryos of 16 mm. the spread into the pericerebral tissues is invariably found. 

 Often, with this extension of the replacement solution outside the ventricles, the 

 oval area noted in the stage of 13 mm. persists. (This phenomenon is especially 

 well shown in a simple injection of silver nitrate, illustrated in figure 11.) 



The next stage of importance in the development of the cerebro-spinal spaces 

 is represented in figure 4, a drawing of a pig embryo of 18 mm. in which a typical 

 intramedullary replacement of the cerebro-spinal fluid with a solution of potassium 

 ferrocyanide and iron-ammonium citrate had been made. Here, with the exception 

 of the region of the roof of the fourth ventricle, the replaced fluid is contained solely 

 within the central canal of the spinal cord and within the cerebral ventricles. The 

 roof region, however, exhibits a new phenomenon, which distinguishes it from the 

 stage shown in figure 3. The chorioid plexus invagination has become strongly 

 developed, dividing the roof into two parts. These roof divisions have been termed 

 superior and inferior, the former lying anteriorly and orally from the chorioid fold. 

 The general surface outline is but little changed, due to the mesenchyme filling up 

 the area between roof and skin. From two areas in the entire roof of the fourth 

 ventricle the foreign fluid has escaped into the pericerebral tissue. These points of 

 fluid passage lie in the two divisions of the ventricular roof. The superior area of 

 escape corresponds to the oval outlined by the prussian-blue in figure 2 and to the 

 point of emergence of fluid shown in figure 3. The lower area of fluid escape is in 

 the inferior half of the ventricular roof, where the ependymal lining and its support- 

 ing tissue are developing into a well-marked dorsal distension. This area corre- 

 sponds to BlakeV 3 ' caudal protrusion, though, as Heuser^ 3 ) has pointed out, the 

 shape of the structure in the pig in no way resembles the "finger of a glove." 



The extraventricular spread of the injection fluid in this specimen is consider- 

 ably greater than in the pig embryo of 14 mm. (fig. 3). On the whole, however, the 

 distribution of the replaced fluid is not extensive as compared with the adult rela- 

 tionship, where the central nervous axis is entirely surrounded by its subarachnoid 

 cushion of cerebro-spinal fluid. From the superior area of fluid passage the replaced 

 solution (as shown by the resultant precipitation of the prussian-blue) has passed 

 both superiorly and inferiorly. In the median line, and extending laterally but 

 slightly, a projection of the blue may be seen occupying a large portion of the 

 extraventricular area formed from the chorioidal invagination. This area of fluid 

 passage occupies at this stage about one-third of the total transverse diameter of the 

 ventricular roof. From it the blue tapers caudally, diminishing in all directions. 

 Above, the precipitate may be made out extending superiorly over the cerebellar 

 lip. Its extension into the pericerebellar tissue is not marked; here again it tapers 

 from the area of fluid passage, its midline prolongation stretching farthest anteriorly. 

 This relationship is easily made out in figure 4, a frank lateral view of such an experi- 

 mental replacement. The granules which result from the introduced ferrocyanide 

 solution are found only in the central canal of the spinal cord and not in any peri- 

 spinal arrangement. 



