DEPARTMENT OF EMBRYOLOGY. 85 



cavity is a clump of spindle-shaped cells, continuous with and similar 

 to the group mesial and posterior to the vagus nerve, which Dr. 

 Shellshear interprets as the migrating elements of the hypoglossal 

 musculature. Through the aid of the Rockefeller Foundation, it was 

 possible for Dr. Shellshear to spend several months of the past win- 

 ter wuth us as a guest. 



Subarachnoid Spaces. 



The series of studies carried out during the past few years by Pro- 

 fessor Lewis H. Weed and other workers associated with him has given 

 us a more definite and concise conception of the morphology and 

 behavior of the cells lining the subarachnoid space, which were pre- 

 viously little understood. In a shorter account, referred to in the last 

 Year Book, Dr. Weed described the changes which these cells undergo 

 with increasing age of the animal, exhibiting in young adult and old cats 

 characteristic proliferations. It will also be recalled that in his mono- 

 graph on the histogenesis of the subarachnoid space he described the 

 development of these cells in the embryo pig. During the past year his 

 observations have been extended and brought together into a complete 

 account, in which he assembles all his previous results. This important 

 paper presents, in the first place, the normal form and disposition of the 

 subarachnoid cells, both in the embryo and in the adult. Dr. Weed 

 follows this with a description of the changes in form which they 

 undergo under physiological activity in response to exposure to various 

 stimulating and destructive substances, including infective processes. 

 Like the mesothelial cells of the peritoneum, in the normal state they 

 spread out in a thin and usually single layer, lining more or less com- 

 pletely the arachnoid membrane and its trabeculse. Under the stimu- 

 lus of injected matter, however, the cells enlarge, become phagocytic, 

 and may break away from their attachments and move about freely 

 as macrophages in the subarachnoid space. Localized proliferations 

 of arachnoid cells, both in man and in laboratory animals, had been 

 previously observed without being understood. As a result of Dr. 

 Weed's observations, it is now clear that they are conditioned by age. 

 They are absent in the young, are usually present in the adult, and 

 increase with advancing age. In tracing their ultimate fate, it is 

 found that they reach a limited size, after which they either persist or 

 undergo degeneration with subsequent infiltration of calcareous 

 material. Occasionally^ they apparently become converted into true 

 endotheliomata. 



Dr. Weed and Dr. Hughson have been able to demonstrate ex- 

 perimentally that the cranium and vertebral canal, within which 

 the central nervous System lies, are so constituted as to form a closed 

 and unyielding chamber and thus, by the administration of hypotonic 

 solutions, it is possible to produce a negative pressure in the cere- 

 brospinal fluid. A preliminary account of these experiments has 

 been given and the complete report is in process of publication. 



