34 NORMAL HISTOLOGY. 



1. Fibrillar connective tissue. 



2. Embryonal and mucous tissue. 



3. Fat tissue. 



4. Reticular connective tissue. 



5. Cartilage. 



6. Bone and teeth. 



Aside from many striking points of similarity in 

 their structure, which we can better appreciate after 

 having made a practical study of the group, three 

 considerations may be briefly noticed here as of 

 weight in determining this classification. 



According to the more recent views of embryolo- 

 gists, especially of His and Waldeyer, the primitive 

 tissues of the animal body belong to two groups : 

 those formed from the archiblast and those formed 

 from the par ~ab 'last. Early in embryonic life, when the 

 animal is still composed of cells, it is found that one 

 of the first definite groupings of these cells consists in 

 the formation of three distinct layers : an outer, the 

 epiblast, a middle, the mesoblast, and an inner, the 

 hypoblast. These three layers are known as the 

 archiblast. From the cells of the epiblast are pro- 

 duced the epithelium of the skin and its adnexa, 

 the epithelium of the terminal portions of the 

 alimentary canal, the nervous system, and the neu- 

 roglia; from the mesoblast are formed the epithe- 

 lium of the genito-urinary organs, smooth and stri- 

 ated muscle ; from the hypoblast the epithelium of 

 the respiratory and digestive organs, glands, and 



