DEPARTMENT OF EMBRYOLOGY. 117 



cells covering the membranous portion of the outer surface of the pia 

 mater undergo the same reaction. All portions show the same degree 

 of response, and the only requisite seems to be actual physical contact 

 with the particulate matter. Where the collections of d6bris are thick, 

 almost every cell shows signs of swelling, while adjoining regions appear 

 relatively quiet. The detachment of the arachnoid cells apparently 

 does not result in a true denuding of the surface, as the protoplasmic 

 bodies of the adjacent cells, which have not detached themselves 

 immediately, close over the gap. The activity in this replacement of 

 cells that have budded off is evidenced by the numerous mitotic figures 

 among the cells covering the trabeculee. Similar experiments were 

 carried on with suspensions of cinnabar and carbon, but phagocytosis 

 was not so vigorous as with the erythrocytes, as the granules are 

 slightly toxic and can not be used for food. The process is consequently 

 slower and it may be months before all the particles are removed. Nor 

 is the number of macrophages so great as in the experiments with laked 

 blood, although one sees the same swelling of the protoplasm and 

 phagocytosis of granules by the cells found on the trabeculse. 



Regarding the fate of those cells which have separated themselves 

 from their normal environment, no additional information was obtained 

 beyond that reported by Quincke, who showed that similar cells con- 

 taining insoluble inert matter slowly wander out along the nerve- 

 trunks and larger vessels. The soluble matter in Dr. Essick's experi- 

 ments with laked blood was promptly digested and only the iron 

 pigment remained. He found no evidence that the free cells, after leav- 

 ing the trabecule, again assumed their former position. 



Reference has been made in a previous report to the work of Pro- 

 fessor Lewis H. Weed on the development of the cerebro-spinal spaces, 

 and his studies showing that the arachnoid vilh are essential structures 

 in the return of the cerebro-spinal fluid into the blood-stream and that 

 the major portion of the cranial subarachnoid space is drained by means 

 of these structures. As a continuation of these investigations Dr. 

 Weed has conducted a series of experiments with a view to an ex- 

 planation of those poorly understood cases of internal hydrocephalus 

 in which the obstruction to the flow of the cerebro-spinal fluid occurs 

 within the subarachnoid space. He has found that it is possible to 

 produce a typical interna,! hydrocephalus by experimentally causing a 

 sterile meningitis. The best results were obtained by injecting into 

 the subarachnoid space, through the occipito-atlantoid ligament, a 

 suspension of lampblack in Ringer's solution. Young kittens in which 

 the bony plates of the skull are not yet united, when treated in this 

 way, rapidly (4 to 10 days) develop typical cUnical symptoms such 

 as are seen in the more chronic cases of this disease in children, the 

 intracerebral pressure being compensated for by a conspicuous en- 

 largement of the head. Similar results were obtained by injecting the 



