32 ORGANIZATION AND DEVELOPMENT OF ANIMALS IN GENERAL. 



the body; the latter, on the contrary, serve for movement and 

 sensation, functions which are exclusively the property of animals 

 (as opposed to plants). For the sake of clearness we will divide the 

 vegetative tissues into two groups, into cells and cell-aggregates 

 (epithelium), and into tissues of connective substance. In the 

 tissues of animal life we distinguish muscular and nervous tissues. 

 This classification of the tissues has no other aim than to facilitate 

 a general review of the different forms of tissue, and to render 

 possible a criticism of their relationships ; it lays no claim to establish 

 an absolutely sharp line between the various groups, 



1. Cells and cell-aggregates. Cells may either be free and isolated 

 from each other, floating in a fluid medium, or they may be placed 

 near one another forming part of an aggregation of cells spread out 

 superficially. To the former belong the cells of the blood, chyle, and 

 lymph. The blood of invertebrates, which is generally colourless, and 



FIG. 19. Blood-corpuscles (af ^er Ecker). a, colourless blood corpuscles from the heart of 

 the fresh water mussel (Anodonta) . b, from the caterpillar of Sphinx, c, red corpuscles 

 from Proteus, d, from the smooth adder, d', lymph corpuscles of the same, e, red 

 corpuscles of the fro^. /, of the pigeon. f t lymph corpuscles of the same, g, red 

 blood corpuscles of man. 



the blood of vertebrates, which is with few exceptions red, consists 

 of a fluid albuminous plasma containing numerous blood-corpuscles 

 in suspension. These corpuscles are in invertebrates irregular often 

 spindle-shaped cells, endowed with the capacity of amoeboid move- 

 ment. In the blood of vertebrates, in addition to such colourless 

 amoeboid corpuscles there are found red corpuscles (discovered by 

 Swammerdam in the frog) ; and these are so numerous as to give 

 the blood a uniformly red appearance to the unaided eye. They are 

 thin discs with an oval, nearly elliptical or circular (Mammalia 

 Petromyzon) contour, with nuclei in the first case, and without 

 nuclei in the eesond (except in the embryo) (fig 19). They contain 



