30 AN INTRODUCTION TO ZOOLOGY. 



A vertebrate affords the most familiar type of highly 

 organized animal j but it must be borne in mind that 

 there are other animals almost equally highly organ- 

 ized, but differing widely in the arrangement of their 

 organs. 



First it must be stated that the young animal in the 

 &^^ (embryo) consists, at a very early stage, of three 

 layers, spoken of as the germinal layers : viz., 

 the outside, or skin layer, called Epiblast (see figs. 

 23 and 24, p. 113) ; the middle, or muscle layer, called 

 Mesoblast ; ;md the inside, or stomach layer (which 

 gives rise to the glandular organs), called Hypo- 

 blast. The first and last are the two primary layers, 

 and the second is formed afterwards. All the parts 

 ■ of the full-grown animal are derived from one or 

 ' other of these layers; and although in the full-grown 

 animal the parts respectively derived from each of the 

 three layers are all mixed up together, j-et it will 

 simplify the student's task to group them according 

 to their origin, which otherwise would have to be 

 learnt separately. 



Parts derived from the Epiblast. The epi- 

 blast of the embryo forms the skin of the adult ani- 

 mal. The most obvious use of the skin is to protect 

 the tissues inside it. For this purpose the cells of its 

 outer layer (epidermis) become loaded with a hard 

 substance; in the vertebrata (back-boned animals), 

 this is a substance called keratin. In the inverte- 

 brata it is a substance called chitin. The latter 

 differs from the former in being more usually associated 



