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



was that in early embryological life the rhombic roof was bordered by a regular, 

 meshed tissue. Later the small meshes in this tissue fused to form the larger fora- 

 men of Magendie. 



Blake's^ hypothesis of the formation of the medial foramen has been quite 

 extensively quoted in the more recent publications on this subject. In a study of 

 the chorioidal roof Blake found a caudal bulging of the inferior velum; this out- 

 pouching became more and more extensive in the older embryos. In man this 

 pouch became sheared off at its neck, leaving the foramen of Magendie. 



In addition to the few studies referred to above, there have been in the past 25 

 years a great number of articles (notably those of Wilder^ 6 ) and Cannieu^ 4 )) offering 

 evidence that this median foramen of the fourth ventricle is an existent, functional 

 opening. Into this literature it is not proposed to go in the present communication; 

 it may be stated that in the larger part the views presented have been in favor of 

 the consideration of the true occurrence of the foramen of Magendie. 



The material on which this study is based has been purely embryological in 

 type, so that no reliable data regarding the foramen of Magendie could be obtained. 

 But even in the largest fetuses examined, there was no evidence which indicated a 

 breaking-down or a shearing-off of the inferior roof of the fourth ventricle. In the 

 largest human fetus at my disposal, in which the histological material was good 

 enough to permit an accurate examination of the chorioidal roof (embryo No. 448, 

 52 mm. in the Carnegie collection) the area membranacea inferior appeared as an 

 intact membrane supported only be a few pial cells. In the pig the material at 

 hand has been such that accurate study of the roof could be made in specimens up 

 to 20 cm. ; in all of these later fetal pigs the roof has been wholly without foramina. 

 If, however, in these larger stages the histological procedures have not been of the 

 best, ruptures and other artificial separations are very frequently found. 



The area membranacea inferior, then, may be regarded as a region of ependymal 

 differentiation. Whether it persists as an intact membrane or undergoes, in certain 

 animals, a perforation to form a foramen of Magendie can not be here answered; 

 this study has been concerned solely with the embryology of the cerebro-spinal 

 spaces, and it affords no evidence in favor of or against the existence of such a fora- 

 men. Nor has any study been made of the two foramina of Luschka, the two open- 

 ings from the lateral recesses of the fourth ventricle into the subarachnoid spaces. 

 It can be stated, however, that these foramina are not in existence at the time of 

 establishment of the circulation of the cerebro-spinal fluid. This phenomenon, as 

 recorded in the previous section, occurs in pig embryos of 26 mm.; at this time the 

 lateral recesses are anatomically and physiologically closed. 



