STRUCTURAL CHARACTERS OF ANIMAL ORGANISMS. 43 



The history, then, of the development of any mammal from a 

 single cell or egg to the complex adult individual, is strictly 

 analogous with the more protracted history of the evolution of 

 the animal kingdom from the Protista upward. 



It is impossible to separate the differentiation of tissues and 

 organs, or to say which is of older date in the history of animal 

 evolution. Even in unicellular animals, where we have no trace 

 of tissue difference (Paramaicium, Vorticella, there being only one 

 cell), we have a distinct foreshadowing of organ and functional 

 differentiation (vide Chapter III). And in creatures made of 

 many parts, the same cells have several duties to perform. But 

 when an aggregation of cell units exists, it may be said that a 

 tissue is formed. If these cells be indifferent, that is, have no 

 special characteristic, then the tissue may be called primitive or 

 embryonic. But, as has just been stated, the aggregation of 

 embryonic cells in the higher forms of life have special char- 

 acters from the very first, which mark them off from one another 

 as destined for different functions. 



The middle germ layer (mesoblast) is derived from the upper 

 (epiblast) and lower (hypoblast), the part contributed by each 

 being doubtful. From the first the middle layer has distinctive 

 characteristics, and ultimately gives rise to a set of tissues which 

 can always be distinguished from those which originate from the 

 upper and lower layers. 



From the inner and outer germ layers are formed several layers 

 of tissues, which, in a more or less perfect degree, retain the 

 activity of the original protoplasm, and hence may be called 

 active tissues. From the middle germinal layer is developed a 

 set of textures, in the majority of which the protoplasmic ele- 

 ments are reduced to a minimum, and are therefore grouped 

 together as supporting tissues. 



The tissues formed in the adult may be classified into four 

 groups : 



1. Epithelial Tissues. The primitive "surface tissues of the 



outer and inner germ layers, which are variously modi- 

 fied for several distinct duties. 



2. Nerve Tissues. Springing from the former, are modified 



