CELLULAR DIFFERENTIATION 59 



80. Nutritive Fluids. — The body fluids known as blood and lymph ate frequently 

 classed among the supporting tissues, the fluid portion being regarded as the inter- 

 cellular substance and the corpuscles as the cells. They differ however from the 

 ordinary tissues in the important fact that the intercellular substance is not pro- 

 duced by the cells. In the vertebrates these cells are of two kinds, the amaboid 

 or colorless and the colored. Both kinds occur in the blood; the colorless alone are 

 found in the lymph. The colored corpuscles are relatively numerous and are disc 

 shaped. Regarded as cells they present a series of degenerative changes which 

 results in a loss of the distinctively protoplasmic character, by the substitution 

 of certain proteid substances, one of which — haemoglobin — is notable for its affinity 

 for oxygen. The degeneracy may go to the extent of the entire loss of the nucleus, 

 as in the mammals. The colorless cells have the power of independent motion 

 such as is found in the amoeba, and may ingest solid particles of food. The body- 

 fluids of the invertebrates contain as a rule only colorless corpuscles, and are there- 

 fore more like the lymph of the vertebrates. When their blood is colored it is 

 usually from pigment in the plasma or fluid portion of the blood. In addition to 

 the cells the blood carries a rich supply of proteid and other substances for use in 

 the tissues, of secretions from the various ductless glands, and of waste products 

 in process of removal from the body. 



81. Muscular Tissue. — The remaining tissues are charac- 

 teristically active. Muscular tissue by its contractility has the 

 power of producing movements of the parts to which it is at- 

 tached. This contractility of muscle may be looked upon as a 

 specialization, and a limitation in direction, of the power of 

 contraction which we have seen to be resident in all living proto- 

 plasm. Muscular tissue differs somewhat in structure and 

 degree of differentiation in various animals, but in general agrees 

 in the presence of elongated fibres which are to be considered as 

 modified cells or parts of cells. The contractile muscle substance 

 is, in part at least, a plasmic product rather than mere proto- 

 plasm; yet it differs from the intercellular substance of the tis- 

 sues already described in that it is deposited within rather than 

 among the cells. 



Two stages in the differentiation of muscular substance are 

 to be noted: (i) the fibres may be plain, in which case we find 

 elongated, contractile single cells without conspicuous external 

 differentiation (Fig. 29); (2) cross-striate fibres, which always 

 show conspicuous differentiation of parts in each fibre as seen 

 under the microscope. The plain fibres are characteristic 

 of sluggish animals, and those parts of animals whose muscular 

 action is least prompt in response to the nervous stimuli {e. g., 

 digestive tract in vertebrates). The cross-striated fibre usually 



