DEPARTMENT OF EMBRYOLOGY. 115 



ing and takes up and digests pabulum (embryotrophe) as well as nutri- 

 tive substances from the mother's blood. Hence the villi of the chorion 

 show an analogy to those of the intestine. The function of destruction 

 and absorption of maternal tissues is not to be ascribed to the sjnicy- 

 tium and giant cells alone, but also to the cells of the trophoblast, for 

 these wander out and come in contact with the maternal cells and no 

 doubt are responsible for their solution. The syncytial giant cells, 

 however, are often found far out in the tissues of the uterus as well as 

 within the maternal blood-vessels. In the latter case they do not de- 

 stroy the maternal cells, but in Professor Keibel's opinion they them- 

 selves are fated to destruction. 



NEUROLOGY. 



Studies on the development of the arachnoid spaces and brain 

 membranes and others on the establishment and character of the circu- 

 lation of the cerebro-spinal fluid in mammalian embryos, have been 

 published in two shorter papers by Dr. Lewis H. Weed. A larger and 

 more comprehensive account of the whole work has been given by him 

 in the Contributions to Embryology, vol. v (Carnegie Inst. Wash. Pub. 

 No. 225). He devised a method of replacing the cerebro-spinal fluid 

 in hving pig embryos with an isotonic solution of potassium ferro- 

 cyanide and iron-ammonium citrate. After the embryos had been 

 kept ahve in an incubator for varying periods, they were fixed in a fluid 

 containing dilute hydrochloric acid. This resulted in the formation of 

 Prussian blue as a granular deposit in all the spaces that had been in- 

 vaded by the injected solution. By the use of this method the cerebro- 

 spinal fluid was replaced by a true solution, without any increase in 

 pressure and without any toxic effect upon the embryos. It was thus 

 possible to obtain trustworthy evidence regarding the distribution and 

 spread of the cerebro-spinal fluid. 



Dr. Weed finds that the first extra-ventricular spread of the cerebro- 

 spinal fluid occurs in embryos over 14 mm., long, and that this is 

 approximately coincident with the development of the tufts in the 

 chorioid plexuses of the fourth ventricle. The spread occurs through 

 a differentiated oval area in the superior central portion of the roof 

 of the fourth ventricle. This area can be recognized histologically 

 before it begins to allow the passage of fluid. In 18 mm. pig embryos, 

 the extra- ventricular spread is by no means extensive; but two areas of 

 escape for the ventricular fluid exist in the two divisions of the rhomb- 

 encephalic roof. The superior of these points of fluid-passage is the 

 one also concerned in the primary outflow of the fluid, whereas the 

 inferior is a somewhat similar area of ependymal differentiation in the 

 caudal half. After this stage is passed, the further extra-ventricular 

 extension of the fluid occurs rapidl}^ The peribulbar spaces are first 

 filled with the fluid, and from this region a spread downward in the 



