DEPARTMENT OF EMBRYOLOGY. 117 



of more fluid into the so-called tissue-spaces. After the length of 

 18 mm. is passed — at which stage a great augmentation in the extra- 

 ventricular spread of the cerebro-spinal fluid occurs— the phenomenon 

 of the disruption of the mesenchymal strands in the peribulbar tissues 

 may be made out. Many of these strands may be observed to have 

 been broken off, sacrificed to a few larger persisting trabeculse. The 

 cells that give rise to these disrupted strands appear to recede, until 

 one of the larger surviving elements is reached, when they adhere and 

 apparently aid in the future production of a permanent arachnoidal 

 trabecula. But associated with this breaking-down of the mesen- 

 chymal mesh, and with this formation of a smaller number of persisting 

 strands, there occurs another phenomenon of great importance. The 

 larger meshes foraied by the process appear to be filled with a fluid 

 much richer in protein than that contained in the original (much 

 smaller) interstices. This fact is proved by the great quantity of the 

 albuminous coagulum found, on histological examination, in all of the 

 enlarged spaces in the tissue. The occurrence of this large amount of 

 albuminous coagulum is apparentlj^ related directly to the distribution 

 throughout this tissue of the embryonic cerebro-spinal fluid; for this 

 embryonic fluid is very rich in protein material, as can be seen by the 

 partial filling of the embryonic cerebral ventricles with the clotted 

 albumin. In this respect the embryonic fluid differs markedly from 

 that of the adult, in which the protein-content is suprisingly low. 



While these spaces are in process of formation the cell-bodies of the 

 disrupted mesenchymal elements adhere to the persisting arachnoidal 

 trabeculse and gradually become transformed into the typical cuboidal 

 mesothelium of the subarachnoid spaces. The very useful term 

 "arachnoid membrane" is introduced to designate the outer intact 

 membrane of the arachnoidea as distinguished from the arachnoid 

 trabeculse. This portion of the arachnoidea first appears as a distinct 

 line of mesenchymal condensation separating the mesenchyme into the 

 primitive arachnoid and dura mater. This rather thin zone of cellular 

 density in reahty represents not only the outer surface of the arach- 

 noidea, but also the inner surface of the dura mater. At first these 

 develop in close fusion, but as the length of 50 mm. is attained in fetal 

 pigs, a separation of the two membranes over the cerebral hemispheres 

 becomes possible. At this stage also a mesothehal polygonal cell- 

 pattern may be made out on the inner surface of the dura by means of 

 silver-nitrate reductions. With this cleavage of the two surfaces, the 

 arachnoid membrane is rapidly differentiated, forming an intact layer 

 over the subarachnoid spaces. The cells covering the surface mem- 

 brane seem to change gradually into cells of cuboidal type, similar to 

 those covering the arachnoidal trabeculse. 



During the year three papers have been published that deal with the 

 embryology of the internal ear. Two of these are on the endolymphatic 



