CONNECTIVE TISSUE. 



21 



derm, while the endothelia are direct modifications of mesodermic cells. The 

 young mesodermic cells bordering the early body-cavity become differentiated 

 into a delicate lining for this space, and later give rise to the plate-like ele- 

 ments which constitute the lining of the permanent serous sacs. The primary 

 lining is known as the mesothelium, which name is often retained to designate 

 the investment of the pericardial, pleural and peritoneal cavities, as distin- 

 guished from the endothelium which lines the vascular and other serous spaces. 



As seen in typical preparations, obtained from the peritoneum after 

 appropriate treatment with argentic nitrate, the endothelial cells on surface 

 view appear as irregularly polygonal areas mapped out by deeply tinted 

 lines (Fig. 25). The latter represent the silver-stained albuminous inter- 

 cellular cement-substance, which joins the flattened cells in a manner similar 

 to that seen in simple squamous epithelium. The lines of apposition are 

 sinuous and less regular than those between epithelial cells, in many cases 

 the lines appearing distinctly serrated. The form of the cells and the char- 

 acter of their contours, however, are not 

 constant, since they are greatly influenced 

 by the degree of tension to which the 

 tissue has been subjected. Protoplasmic 

 threads directly connecting the adjoining 

 cells have been described. 



After silver staining the intercellular 

 substance frequently shows irregular, 

 deeply colored areas at points where 

 several endothelial cells come together. 

 These figures are described as stigmata 

 or pseudostomata and by some are inter- 

 preted as indications of the existence of 

 minute openings leading from the serous 

 cavity into the subjacent lymphatics. They are, however, largely accidental 

 and due to dense local accumulations of the stained intercellular materials. 

 True orifices, or stomaia, on the other hand, undoubtedly exist in certain 

 serous membranes, as in the septum between the peritoneal cavity and the 

 abdominal lymph-sac of the frog and, probably, the peritoneal surface of the 

 diaphragm of mammals. The positions of these stomata are marked by a 

 conspicuous modification in the form and arrangement of the surrounding 

 endothelial plates, which are radially disposed about centres occupied by the 

 stomata. The immediate walls of the orifices are formed by smaller and 

 more granular elements, the guard-cells, the contraction and expansion of 

 which probably modify the size of the openings. 



THE CONNECTIVE TISSUES. 



The important group of connective substances, the most widely distrib- 

 uted of all tissues, is the direct product of the middle germ-layer. Since the 

 latter is also the origin of epithelial, muscular, vascular and lymphoid tissues, 

 that portion of the mesoderm especially concerned in producing the connec- 

 tive tissues has been designated the mesenchyma. Their essential role, con- 

 nection and support, being largely passive and mechanical, the physical 

 characteristics of these tissues are of much importance. These depend upon 

 the intercellular substance, which, in marked contrast to the meagre cement- 

 substance of the epithelia, is very large in amount and contributes the chief 

 bulk of the tissue. 



FIG. 26. Endothelial cells lining artery of 

 dog ; stained with silver and hematoxylin. 



X45. 



