8o COMPARATIVE ANATOMY 



degree from one another. The leucocyte is comparatively 

 formless, and its characters are such as to afford an excellent 

 illustration of what we mean when we speak of an animal cell. 

 It is, in fact, an embodiment of our idea of a cell, and is there- 

 fore what we call typical or generalised. The hstmatid, on the 

 other hand, has characters all its own. Its structure departs 

 from the general conception, because it is altered in connection 

 with its function as a carrier of haemoglobin, and it is what we 

 call specialised or differentiated. Its cell-body is not simple 

 protoplasm, but is in large part changed into something that 

 is not protoplasm but has been formed by or out of protoplasm. 

 This circumstance shows us that a cell has activities, that its 

 constituent protoplasm has the power of changing itself into 

 some special substance, or of manufacturing some special 

 substance, and depositing it within the limits of the cell. A 

 cell, therefore, has formative powers, in virtue of that mysterious 

 attribute of its constituent protoplasm which we call life. Life 

 is the sum-total of the activities of the cell. 



The truth of this statement is borne in upon us when we 

 examine further the constitution of the other tissues of the 

 frog's body. All of them, skin, bone, cartilage, nervous tissue, 

 muscular tissue, gland tissue, are formed either of cells, or of 

 the union of the products of cells. But there are few of the 

 constituent cells of the frog's body which are as simple and 

 generalised as the leucocytes of its blood. 



We may first of all study the composition of the membranes 

 which cover the surface or line the cavities of the body. 

 These are called epithelia ; and, since all epithelia are com- 

 posed of cells, the latter are known as epithelial cells. The 

 most simple kind of epithelium is perhaps that variety known 

 as columnar. Columnar epthelium is found in the intestine 

 of the frog, lining its cavity, and forming the coverings of the 

 minute processes, called villi, which project into the cavity. 

 Each component cell of a columnar epithelium is a prism 

 of greater or less height, the body of the prism being 

 cytoplasm, in the interior of which lies a nucleus. These 

 prismatic cells usually rest on a very fine basement membrane, 

 which is not cellular, but produced by the cells. The prisms 

 form a single layer, are pressed close together, and owe their 

 prismatic shape to mutual pressure. They are not so closely 

 packed, however, but that there are minute intercellular spaces 



