42 



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



bis zum Begiiin des dritten Monats," shows a slight irregularity in the roof. Prac- 

 tically all of the contributions to the anatomy of the roof of the fourth ventricle 

 deal with the lower half of the structure, with particular reference to the occurrence 

 of the foramen of Magendie. 



The general biological process involved in the formation of the area membra- 

 nacea superior concerns a differentiation of the epidermal elements which line the 

 ventricular cavity. This differentiation, both in human and in pig embryos, first 

 begins with the occurrence in the ventricular roof of an area of epithelial-like cells. 

 These, in the course of enlargement of the roof, become more or less isolated in the 

 superior portion of the structure, and then undergo a metamorphosis into the typical 

 cells of the membranous area. They are characterized by oval or elongated nuclei 

 (rather poor in chromatin as compared with the nuclei of the typical ependymal 

 elements) and by cytoplasmic strands (in which the cell-boundaries are very poorly 

 marked) which compose the ventricular border. The ventricular surface in the 

 area membranacea is more ragged and irregular than where lined by typical epen- 

 dyma. In many instances, as in figure 57, from a human embryo of 14 mm., this 

 transformation has proceeded to such an extent that the epithelial character of the 

 lining cells is almost wholly lost, and the ventricle seems, in this area, to be lined 

 by mesenchyme. Study of the membranous area in many stages convinces one 

 that such an hypothesis is untenable; in every case the ventricle must be considered 

 as being lined by epidermal elements, no matter to what extent the process of differ- 

 entiation has proceeded. There is no real evidence to support the view that the 

 ependymal lining of the ventricle has been replaced by mesenchymal elements to 

 form the area membranacea superior. 



In general the area membranacea superior is a rounded oval. Its measurement 

 is quite difficult except when fixation and dehydration have been excellent, because 

 of the highly abnormal distortion of the ventricular roof which frequently occurs in 

 the technically poor specimens. Measurements have been made in a considerable 

 number of favorable specimens, both of human and pig embryos. With the history 

 of this area in mind, it will be realized that the size of the structure necessarily varies 

 with the length of the embryo, attaining its greatest dimensions at about the length 

 of 18 or 20 mm. Herewith is a short table of the measurements taken. 



Dimensions of area membranacea superior. 



In a rough way, then, we may consider the area membranacea as an oval; in 

 some cases the longitudinal diameter exceeds the lateral, and in others the reverse 



*Measured on slide after mounting. 



