I30 



COMPARATIVE ANATOMY 



animal are the gastrular layers persisting in their original relations to 

 space. By far the greater part of the massive adult has been inserted 

 between the two primary layers. 



In view of the foregoing facts, the significance of the outer and inner- 

 most epithelia of the body and their immediate derivatives is such that 

 attempts have been made to recognize it in terminology. The embryolo- 

 gist, Wilhelm His, in 1865, proposed to apply the term endothelium to all 

 mesodermal limiting layers, thus restricting "epithelium" to the ectoder- 

 mal and endodermal epithelia and their immediate derivatives. At 

 present, however, the term endothelium is rarely used except to designate 

 the lining layer of blood vessels and lymphatic vessels. To designate the 

 mesodermal layer lining the coelom, that is, the peritoneal epithelium, 

 the term mesothelium has been proposed. 



In present usage, the term endothelium may be correctly appHed to the 

 lining layer of blood vessels and 

 lymphatics. "Mesothelium" may 

 be used for the peritoneal 

 epithelium. 



In certain localities are found 

 tissues whose cells, in their appear- 

 ance and arrangement, resemble 



Fig. 91. — Types of epithelia. B, simple squamous; C, simple columnar; D, strati- 

 fied columnar, ciliated at E\ F, stratified polyhedral, upper cells squamous. (From 

 Kingsley.) 



epithelial cells, but they do not abut upon a cavity. The insuhn- 

 producing ''islands" or "islets" in the pancreas, the deeper tissue or 

 medulla of the suprarenal gland, and the tissues of some other endocrine 

 glands are of this nature. These are glands whose secretion is not 

 collected in a cavity but transmitted directly to the blood in closely 

 adjacent vessels. To such tissues, which are not typically epithelial, is 

 applied the adjective epithelioid. 



The most notable exception to the rule that all free surfaces of the- 

 animal are epithelial is found in lymph glands whose irregular internal 

 spaces do not have a continuous endothelial covering. Its absence is 

 intelligible. Cells proliferated from the spongy deep tissue of these glands 

 float away in the lymph stream to become new white blood cells. An 



