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



negie Institution. In this specimen (fig. 92) the fourth ventricle seems almost to 

 lack a lining of ependymal (epidermal) elements in the area membranacea inferior 

 (ami). The cells of this area are small, inconspicuous in their distinctions from the 

 underlying mesenchyme. The whole character resembles that of the superior area 

 membranacea shown in figure 57. 



The appearances exhibited by the inferior membranous area in the stages above 

 26 mm. are modified in great part by the development of the great cisterna cerebello- 

 medullaris. As in the pig, the breaking-down of mesenchyme to form this cistern 

 results finally in the almost total isolation of the inferior membranous area. The 

 cistern is fairly rapidly formed when once the process begins, and so in an embryo of 

 35 mm. (No. 199 in the Carnegie collection) the isolated character of the area mem- 

 branacea inferior (ami) may be easily made out. This is shown in figure 94, an 

 enlargement of the blocked area in figure 93. The general architecture of the mem- 

 brane, particularly its intact character, appears in this photomicrograph, but its 

 finer structure is obscured by the albuminous coagula which adhere on both sur- 

 faces. The cell structure of the area membranacea resembles closely that described 

 in the embryos already pictured. 



Discussion of the final disposition of the area membranacea inferior will be 

 undertaken in the following subdivision of this paper, in order that the findings in 

 the pig and in the human embryo may be correlated. 



GENERAL CONSIDERATION OF THE AREA MEMBRANACEA INFERIOR. 



The ependymal lining of the caudal portion of the roof of the fourth ventricle 

 undergoes a process of differentiation which results in the formation of the area 

 membranacea inferior. This transformation has been observed in pig and human 

 embryos; in both, the first definite evidence of the cellular change has been observed 

 in specimens of 15 mm. The essential phases of the process are identical in the 

 two embryos. The tendency of the deeply staining typical ependymal elements is 

 to lose their highly pyknotic character; the nuclei become poorer in chromatin 

 and the cytoplasm somewhat more abundant. In the first stages of the metamor- 

 phosis the lining cells come to assume epithehal-like appearances, but in the final 

 change the nuclei become small oval bodies, poor in chromatin, resembling to some 

 degree the nuclei of the adjoining undifferentiated mesenchyme. In the human 

 embryo, a tendency for the epithelial-like characters to persist in isolated cellular 

 aggregations is apparent. 



After the initial process of differentiation has begun, the area membranacea 

 inferior increases rapidly in extent and the differentiated cells which characterize 

 it come to occupy the greater portion of the caudal part of the chorioidal roof. In 

 the somewhat later stages the area membranacea is almost wholly unsupported by 

 other tissues, due to the development of the cisterna cerebello-medullaris. As soon 

 as the cistern forms, the area membranacea serves as practically the sole dividing 

 membrane between the ventricular system and the future subarachnoid spaces. 



