CONTAINED PERIOTIC TISSUE-SPACES IN THE HUMAN EMBRYO. 45 



not be definitely determined by observations of fixed tissue; but the appearance 

 of sections where the process is in active operation seems to the writer to indicate 

 the latter. 



In the above paragraph and elsewhere in this paper reference is made to 

 trabeculaj serving as "partitions" between "spaces" and the disappearance of 

 trabeciilse resulting in the "coalescence of adjacent spaces." In making this use 

 of the term "space" it should be explained that it is done in a descriptive sense, in 

 application to the appearance of the tissue as seen in sections in which form human 

 embryological material is mainly available. In thin sections of a reticular tissue 

 one sees trabeculaB as partitions separating adjacent spaces. The same tissue in 

 a mass would show that the spaces everywhere communicate freely with each other, 

 like the spaces in a sponge, and that the trabeculae are thread-like strands which 

 at the best are very incomplete partitions. Instead of a meshwork containing 

 many small spaces, one could perhaps equally well describe reticular tissue as a 

 single large space traversed by many trabeculai. If the latter practice were adopted, 

 one would describe the development of the tissue-spaces with which we are con- 

 cerned as a process of gradual decrease in the number of traversing trabeculse, 

 with the result that the mesh thereby becomes coarser. For descriptive purposes, 

 however, it is convenient to refer to the intervals between the strands of the mesh 

 as spaces, at the same time not granting them the significance that is attached to 

 such membrane-lined tissue-spaces as are represented by the vestibular cistern 

 and the two scalse, though the latter are in reality derived from them. 



In figure 23 the free detached ends of the trabeculse will be noted everywhere, 

 as is characteristic of this stage of development. It is a necessary step in the coa- 

 lescence of adjacent spaces. The detached trabeculse seem to be gradually retracting 

 and adapting themselves to the formation of larger spaces. Their constituent 

 protoplasm reshapes itself as a smooth border or as a part of other trabeculaB. 

 Larger spaces necessitate longer trabeculse, and as. trabeculse become longer they 

 also tend to become heavier. These phenomena are all in evidence in the spreading 

 and enlargement of the scalse. 



Figure 20 shows a characteristic view of the scalse as seen under low mag- 

 nification. It will be noted that the scala vestibuli is relatively mature at this 

 point; the scala tympani, however, is in the act of spreading peripherally, so as to 

 underlie, as it eventually will do, the future basilar membrane. The scala tympani 

 finally reaches the peripheral margin of the cochlear duct, and it does this by the 

 coalescence of the enlarging reticular spaces, which become incorporated with the 

 main cavity of the scala. This can be observed better in figure 22, which shows a 

 detail of the same section as seen under higher magnification. By comparing this 

 figure with figure 20 the exact location can be readily made out. A portion of the 

 main cavity of the scala is indicated and to the right of this are a few enlarged 

 reticular spaces that are uniting with each other and will in the end become part 

 of the main space. In addition to the enlarged reticular spaces there is a certain 

 amount of residual undifferentiated reticulum. It is this tissue that will play the 



