HISTOLOGY 



127 



Histology concerns itself with the internal and specific structure and 

 organization of tissues. Since the tissue is constituted of cells, histology 

 is necessarily concerned with those specific intracellular differentiations 

 which collectively create a tissue. 



C3rtology, narrowly defined, deals with cells as such — that is, with 

 that fundamental cell mechanism which is common to all cells and inde- 

 pendent of tissue specialization. In practice, however, it is impossible 

 to maintain a sharp distinction between cytology and histology. The 

 anatomist, too, never hesitates to become, upon occasion and temporarily, 

 a histologist or a cytologist. 



Most vital functions involve the surface between protoplasm and the 

 medium external to it. Food enters from without. Respiratory gases pass 

 in and out. Waste is expelled from the surface. Special secretions are 

 produced at the surface. External forces impinge upon the surface and 

 give rise to internal progressive changes (nerve conduction) which may 

 result in a reaction directed toward the exterior. Consistently with these 

 facts, as the size of animals increases above that of the unicellular proto- 

 zoans the substance of these larger animals is so disposed as to present, 

 directly or indirectly, an adequate amount of surface to the exterior. 

 Indefinite increase in size of cells and nuclei is rendered impossible by 

 the necessity of maintaining appropriate relations of cell surface to cell 

 volume as well as of nuclear surface to nuclear volume, for the necessary 

 interaction of nucleus and cytoplasm must take place through the nuclear 

 membrane. Therefore the protoplasm of the larger animal must be 

 subdivided into many cells. 



In the preceding chapter on embryology it was pointed out (page 123) 

 that a large and powerful animal necessarily possesses various specialized 

 organs. To say that an animal is large because it has many organs and 

 is complex is the reverse of the truth. Attainment of relatively large 

 size together with capacity for powerful movement and diversified activi- 

 ties compels the differentiation of various organs and results in a high 

 degree of anatomical and histological complexity. Protoplasm being 

 what it is and under necessity of maintaining certain surface relations with 

 the exterior, larger animals must be multicellular and must be complex. 



Among the coelenterates are found the simplest of multicellular 

 animals, consisting of two layers of cells, an outer which is essentially 

 protective and an inner which is nutritive. The double wall encloses a 

 cavity which has a single external opening for admission of food and 

 expulsion of waste. 



The gastrula is a form attained by the early embryo of nearly all 

 metazoan animals, both invertebrate and vertebrate. At the gastrula 

 stage all of the embryonic material is arranged in two layers, an ectoderm, 

 prospectively protective and nervous, and an endoderm, nutritive. The 



