72 HUMAN ANATOMY. 



these figures are described as stigmata or pseudostomata, and by some are interpreted 

 as indications of the existence of openings leading from the serous cavity into the 

 subjacent lymphatics. Critical examination of these areas, however, leads to the 

 conclusion that they are largely accidental, and due to dense local accumulations of 

 the stained intercellular materials ; they are not, therefore, to be regarded as intercellu- 

 lar passages. True orifices or stomata, however, undoubtedly exist in certain serous 

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

 lymph-sac of the frog, and, possibly, 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 exhibit a 

 radial disposition about the centres occupied by the stomata. The immediate walls 

 of the orifices are formed by smaller and more granular elements, the guard or ger- 

 minating cells, the contraction and expansion of which probably modify the size of 

 the openings. 



Although the ectoblast and the entoblast are the germ layers which furnish great 

 tracts of epithelium in the adult body, yet the mesoblast, the middle germ layer, 

 also supplies distinct epithelial tissues. As it has been already pointed out, the 

 epidermis,, the epithelial portion of the skin, with its derivatives, is a product of the 

 ectoblast. The epithelial lining of the mouth cavity as far back as the region of the 

 palatine arches, and the epithelium of the anus are also of ectoblastic origin, since 

 they are formed as in-pocketings of the outer germ layer during early embryonic 

 life. With the exception of these areas, the epithelium lining the entire digestive 

 tube, and that of its accessory glands, notably the liver and the pancreas, is of 

 entoblastic origin. The same thing is true of the epithelium of the respiratory tract, 

 since this entire tract is an outgrowth from the primitive intestine. But in the case of 

 the uro-genital system, the epithelium there found, or most of it, is derived directly 

 from the mesoblast. To be more specific, the Fallopian tubes (uterine tubes), 

 uterus and vagina of the female, which have, of course, a distinct layer of epithelium 

 on their inner surface, are formed from certain embryonic tubes known as the 

 Miillerian ducts, which are derived from the mesoblast. The vas (ductus) deferens 

 of the male is first represented in the embryo by a tube known as the Wolffian duct, 

 which, with its epithelium, is also derived from the mesoblast. The sex-cells found in 

 the sex -glands, which in the case of the male retain a distinct epithelial character, 

 are apparently of mesoblastic origin. The ureter and part of the kidney are out- 

 growths from the Wolffian duct and therefore mesoblastic, while the rest of the 

 kidney not formed in this way is also of mesoblastic origin. Hence, it is evident 

 that distinct layers of epithelium are formed from all three germ layers, and that 

 in this respect no peculiarity is attributable to any one of them. 



