THE ELEMENTS OF STRUCTURE. 



When critically examined, the various organs and parts going to make up the 

 complex economy of the most highly specialized vertebrate — and, indeed, the same 

 is true of all animals whose organization does not approach the extremely simple uni- 

 cellular type — are found to be constituted by the various combinations of a very 

 small number of elementary tissues ; these latter may be divided into four funda- 

 mental groups : 



Epithelial tissues ; 



Connective tissues ; 



Muscular tissues ; 



Nervous tissues. 



Of these the nervous tissues are most specialized in their distribution, while the 

 connective tissues are universally present, in one or another form contributing to the 

 composition of every organ and part of the body. The tissues of the circulatory 

 system, including the walls of the blood-vessels and lymph-channels and the corpus- 

 cular elements of their contained fluids, the blood and the lymph, represent special- 

 izations of the connective tissues of such importance that they are often conceded 

 the dignity of being classed as independent tissues ; consideration of the develop- 

 ment of the vascular tissues, however, shows their genetic relations to be so nearly 

 identical with those of the great connective-tissue group that a separation from the 

 latter seems undesirable. 



Each of the elementary tissues may be resolved into its component morphologi- 

 cal constituents, the cells and the intercellular substances. The first of these are the 



Fig. 4. 



Nucleus Vacuole 



Pseudopod 



Vegetal food- 

 inclusions 



Exoplasm 

 .^, unicellular animal (amaba); .ff. embryonal cell, — leucocyte. 



Endoplasm 



descendants of the embryonal elements derived from the division or segmentation of 

 the parent cell, the ovum, and are highly endowed with vital activity ; the intercellu- 

 lar substances, on the other hand, represent secondary productions, comparatively 

 inert, since they are formed through the more or less direct agency of the cells. The 

 animal cell may exist in either the embryonal, matured, or metamorphosed condition. 

 The embryonal cell, as represented by the early generations of the direct off- 

 spring of the ovum, or by the lymphoid cells or colorless blood-corpuscles of the 

 adult, consists of a small, irregularly round or oval mass of finely granular gelati- 

 nous substance— the /rc'/^/>/«.yw« — in which a smaller and often indistinct spherical 

 body — the nucleus — lies embedded. In the embryonal condition, when the cell is 

 without a limiting membrane, and composed almost entirely of active living matter, 

 the outlines are frequently undergoing change, these variations in shape being known 

 as amceboid movements, from their similarity to the changes observed in the outline 

 of an active amoeba, the representative of the simplest form of animal life, in which 



