HISTOLOGY 129 



horn-like layer which prevents abrasion of the walls of the organ when its 

 powerful muscles grind up hard seeds with the aid of ingested pebbles. 



Provision for the adequate carrying on of these diverse and important 

 surface activities can be afforded only by the presence of a superficial 

 membrane constituted of living material and specialized appropriately 

 for the functional requirements of the particular surface. Tissue highly 

 specialized for motor, skeletal or nervous functions could not at the same 

 time meet the requirements of a surface layer. Consequently, with very 

 rare exceptions, every free surface of an animal, external or internal, is the 

 surface of a more or less specialized cellular layer, an Epithelium. 



EPITHELIAL TISSUES 



In view of the foregoing considerations, it is clear that epithelia are 

 tissues of primary importance. They are the immediate agencies con- 

 cerned in the necessary interactions between protoplasm and the materials 

 and agencies external to it. They provide a barrier against invasion by 

 disease-producing organisms. They are important also in serving to seal 

 the surfaces of the animal against leakage of the omnipresent intercellular 

 lymph of deeper tissues. If human skin is abraded so slightly as not to 

 cause bleeding, nevertheless a clear watery fluid, lymph, exudes. The 

 outer portion of an intact epidermis prevents such leakage. 



Epithelia are, in double sense, the most primitive of tissues. The 

 smaller simpler coelenterates consist merely of an outer and an inner 

 epithelium. There is much reason for believing that all other metazoans 

 are descended from some ancient and primitive coelenterate. The gastrula 

 of animal embryos consists of two epithelia. All tissues of the adult are 

 derived from the primary ectoderm and endoderm. Hence, presumably in 

 phylogeny and certainly in ontogeny, all tissues are derived from epithelia. 



It is evident, therefore, that epithelium provides for all animal needs, 

 and therefore all-epithelial animals may and do exist. But there are no 

 non-epithelial animals — unless protozoans be considered as such, but even 

 a unicellular protozoan may be regarded as a minimum epithelium, being 

 one cell thick and one cell in extent ! 



The outer layer of the gastrula, while it is the source of various gland- 

 ular and other structures which attain a deeper position, otherwise persists 

 as the epidermis which is the external epithelium of the adult body. The 

 inner layer of the gastrula, which gives rise to various organs such as the 

 liver, pancreas and lungs which grow outward from the enteron, otherwise 

 persists as the lining of the digestive tube, the digestive epitheliimi, which 

 is the innermost epithehum of the adult body. These two layers, then, 

 the very thin epidermis and the even thinner digestive epithelium, which 

 together constitute only an extremely small fraction of the bulk of the 



