AGE OF THE TISSUES. 361 



constitute special sexual organs in a morphological sense; 

 they are rather, as we shall soon see, simple component parts 

 of other organs. 



Like the organ-systems of the human body, the tissues, 

 which compose these systems, are of different ages and of 

 vaiying morphological value. As we were justified in 

 drawing an inference as to the phylogenetic sequence in 

 age of the organ-systems, from the ontogenetic sequence 

 in which they successively appear in the embryo, so are 

 we justified in inferring the order in which the tissues 

 originated during the course of tribal history, from the 

 sequence of the stages in germ-history. The result of this 

 is a phylogenetic classification (Table XXXVIII.) of the 

 tissues of the human body, similar to that of the organs 

 (Table XXXIX., p. 367). 



The tissues of the human body, arising by division of 

 labour, the separation and the connection of the component 

 cells, may be distributed, with reference to their develop- 

 ment, in the four following distinct groups : 1, covering- 

 tissue (epithelium); 2, connective tissue (connectivum)', 3, 

 nerve and muscular tissue (neuro-muaculum) ; and 4, vas- 

 cular tissue (vasalium). Of these, in accordance with the 

 Gastnea theory, we must regard the covering-tissue as the 

 oldest and most original form, as the actual primary or 

 primitive tissue ; the three other main forms must, on the 

 other hand, be considered as secondary or derived forms, 

 which developed at a later period from the covering-tissue ; 

 the connecting-tissue first, then the neuro-muscular, and 

 lastly the vascular tissue. 



The oldest and most original form of tissue is, un- 

 doubtedly, the covering-tissue (epitlieliuTri), the cells of 



