14 



INTRODUCTION TO CYTOLOGY 



Cells specialized in connection with motility, such as spermatozoa 

 and the cells of certain epithelial tissues, show complex structural modi- 

 fications not only in the flagella, cilia, and cirri which they bear but 

 also in the other cell organs with which the activities of these motile 

 structures are closely connected (Figs. 134, 136, 137). Secretory cells 

 are often distinguishable by the accumulations of secretion products in 

 their cytoplasm, or by the peculiar form assumed by their nuclei (Fig. 22). 



In connective tissues (Fig. 10) the protoplasm is subdivided into cells 

 with various degrees of distinctness depending on the relative amount 

 of supporting substance they produce during their differentiation. 



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Fig. 9. — Nerve and muscle cells of animals. A, diagram of a typical neuron: a, axis 

 cylinder process, or axon, ending in arborescent system; d, dendrites. B, cell from human 

 spinal cord. {A and B after Obersteiner and Hill.) C, nerve cell from eye. {After Len- 

 hossek.) D, nerve cell from earthworm. {After Kowalski.) E, young voluntary muscle 

 cell. F, portion of mature voluntary muscle cell, showing striations. G, involuntary 

 muscle cell from intestine. {E-G after Piersol.) 



Cartilage and bone cells are likewise imbedded in such substances, which 

 are here produced in relatively enormous amounts and later form the main 

 supporting framework of the body. Blood, which is sometimes spoken of 

 as a "fluid tissue," consists of a plasma carrying cells of a variety of 

 types. 



Differentiation in a multicellular mass involves the gradual setting 

 apart of special regions with modified structural and functional charac- 

 ters as in the body of a protozoon, but with the important difference that 

 these regions include many cells, each of which retains in some degree the 

 fundamental type of protoplasmic organization (nucleus, cytoplasm, 

 semipermeable membranes) possessed by the cell which began the 

 development of the differentiating mass. One therefore expects a greater 

 capacity for independent action in these cellular components of the body 

 than in the subcellular components of the protozoan body. This expec- 



