CONTAINED PERIOTIC TISSUE-SPACES IN THE HUMAN EMBRYO. 4', 



period undergo proliferation and occupy the space vacated by the receding pre- 

 rartilage. In the later stages, however, it is quite evident that precartilage tissue 

 is actually converted into a reticulum, and that the replacement of precartilage 

 by a reticular connective tissue is brought about through a process of dedifferen- 

 tiation. 



The perichondrium is a derivative of the periotic reticulum and forms an outei 

 limiting membrane along its cartilaginous margin. During the fetal period the 

 perichondrium does not rest directly against the true cartilage, but is separated from 

 it by a zone of transitional tissue consisting partly of precartilage and partly of 

 reticulum. This transitional zone intervening between the perichondrium and the 

 surrounding cartilage was observed in all of the specimens that were studied, which 

 includes fetuses up to 130 mm. crown-rump length. Owing to the fact that the 

 perichondrium is late in making its appearance, being first seen in fetuses about 

 70 mm. long, it can take no part in the early changes in the cartilaginous capsule, 

 either as regards the deposit of new cartilage or the excavation of cartilage that 

 had been previously laid down. 



The periotic tissue-spaces are formed by a modification of the meshes of the 

 periotic reticulum. The latter consists originally of a rather uniform narrow mesh. 

 The essential change which it undergoes in the process of space-formation consists 

 in the gradual disappearance of the traversing trabecula?. The trabeculse consist 

 of the protoplasmic processes of the constituent cells of the reticulum, and their 

 disappearance is apparently due, not to a dissolution or liquefaction of these cell- 

 processes, but to an alteration in their form. It apparently is the result of an active 

 motility of the cell protoplasm involving the successive detachment and retraction 

 of the trabeculse. When a trabecula becomes detached it retracts and adapts 

 itself to the formation of the enlarging space, reshaping itself either as a smooth 

 border or as a constituent part of another trabecula. 



The differentiation of the margin of the periotic spaces constitutes the final 

 feature in their maturation. During the period in which the enlargement of an 

 individual space is actively going on, the margins of the main cavity consist of 

 smooth, delicate strands of nucleated protoplasm that resemble the trabeculse 

 between the large reticular spaces. These linear margins are interrupted here and 

 there by openings into adjacent spaces. They tend, however, to form a continuous 

 line that definitely marks off the space from the adjacent reticulum. As the space 

 becomes more mature, the membrane-like border becomes thicker until it reaches 

 a state that will probably not admit of any further opening-up for the coalescence 

 of additional spaces. Any further growth is thereafter limited to a simple distent ion 

 of the wall of the space, with the consequent adjustment of its constituent cells. 

 In its final form the margin of the space constitutes a mesothelial membrane. 

 Immediately lining the space is a thru membrane with flattened nuclei which is 

 supported underneath by a thin coat of nucleated protoplasm having the form of 

 fibrous connective tissue. The former in its histogenesis differs in no way from 

 the rest of the wall and the difference that exists later seems to be merely the 

 result of its adaptation to the existing physical conditions. 



