388 HAL DOWNEY 



determine the boundary between the hypodermis and the connec- 

 tive tissue, and the supporting fibrils are continuous from one 

 tissue to the other. In the deeper portions of the connective 

 tissue syncytium the fibrils pass to the margins of the syncytial 

 strands, thus entering into the structure of true Leydig's cells of 

 the first order. The fibers or bands are continuous from one 

 Leydig cell to another but are always located at the margin of 

 the cells. Some of the fibers join the outer adventitial membrane 

 of the blood vessels the wall of which is composed of Leydig's 

 cells which are so arranged that they form simple epithelial tubes 

 which have a very definite inner and outer membrane. The inner 

 membrane appers homogeneous, but it has the same structure 

 and staining reactions as the fibers of the hypodermis and support- 

 ing tissue. The outer membrane is frequently fibrous, its fibers 

 being continuous with those of the surrounding Leydig's cells. 



That the hypodermis is continuous with the underlying sup- 

 porting tissue is not a new observation. Leydig, in 1851, called 

 attention to the similarity between Arthropod integument and 

 connective substance and M. Braun, in 1875, states that it is 

 impossible to separate the products of the ectoderm from those 

 of the mesoderm in the crayfish, and many other authors have 

 made the same observation. In view of these facts it is not sur- 

 prising to find some of the fibrils from the supporting tissue joining 

 the horizontal layer of hypodermal fibers (figures 1 and 2, of which 

 S. T. is supporting tissue). We must admit, therefore, that it 

 is quite possible that connective fibrils from the supporting tissue 

 take part in the formation of the fibrous limiting layer where such 

 a structure is formed but, on the other hand, it is also possible 

 that the fibers which we see running down into the supporting 

 substance are derived from the hypodermis. The presence of 

 occasional flattened oval nuclei among the horizontal fibers has 

 been interpreted by Schneider and others as proof for the existence 

 of connective tissue in this region. Further speculation on this 

 point is useless, especially so in view of the fact that we are by no 

 means certain of the mesodermal origin of the supporting tissue 

 or of the muscles in Arthropods. The fact remains, that in the 

 crayfish, the layer of horizontal fibers is composed largely of hypo- 



