PH. II.] KPITFTELITM. 23 



The epMast, the outermost layer of the embryo, forms the 

 epidermis, the outermost layer of the adult It also forms the 

 nervous system. 



The hypoblasty the innermost layer of the embryo, forms the 

 lining epithelium of the alimentary and respiratory tracts, that 

 is, the innermost layer of the adult. It also forms the cellular 

 elements in the large digestive glands, such as the liver and 



Fig. 27. Diagram of an ovum (a) undergoing segmentation. In (b) it has divided into two, 

 in (e] into four ; and in d the process has resulted in the production c f the so-called 

 "mulberry-mass." (Frey.) 



pancreas, which are originally, like the lungs, outgrowths from the 

 primitive digestive tube. 



The mesoblast forms the remainder, that is, the great bulk of 

 the body, including the muscular, osseous and other connective 

 tissues ; the circulatory and urino-genital systems. 



CHAPTER III. 



EPITHELIUM. 



IN the introductory chapter will be found a list of the 

 elementary tissues of which the organs of the body are built up. 

 These may be arranged into the four groups, epithelial, connec- 

 tive, muscular, and nervous. The first of these, the epithelial 

 tissues, follows naturally on a study of the animal cell, as an 

 epithelium may be defined as a tissue composed entirely of cells 

 united by a minimal amount of cementing material. As a rule, 

 an epithelium is spread out as a membrane covering a surface or 

 lining the cavity of a hollow organ. 



These epithelia may be grouped into two great classes, each 

 of which may be again subdivided according to the shape and 

 arrangement of the cells of which it is composed. The following 

 table gives the principal varieties : 



