10 



INTRODUCTION TO CYTOLOGY 



Mac. 



Mic 



Differentiation. — Differentiation occurs in uninucleate cells, multi- 

 nucleate Plasmodia, and multicellular masses. In the bodies of certain 

 Protozoa (Fig. 5) one sees within a single cell a very elaborate regional 

 differentiation in structure, certain functions being localized in definitely 

 constituted organs. There are distinct locomotor, digestive, and excre- 

 tory systems, as well as a "neuromotor apparatus" 

 which functions much as does the nervous system 

 of larger animals.^ These are probably the most 

 complex cells known. ^ 



Differentiation in multinucleate masses of proto- 

 plasm may be seen in the plasmodia of myxomy- 

 cetes, in the coenocytic algse and fungi, and in the 

 embryos of certain higher animals and plants. In 

 the myxomycete the protoplasm heaps up locally 

 and develops into a highly characteristic fruit body 

 with well-differentiated stalk, capsule wall, and sup- 

 porting capillitium filaments, but there is no sub- 

 division into cells until the spores are finally 

 delimited. In the coenocytic algae, such as Vaucheria 

 and Caulevpa, the body develops a definite form and 

 structure without cellular subdivisions. In Cau- 

 lerpa (Fig. 6) there are well-formed "roots," 

 "stems," and "leaves," yet the whole vegetative 

 body is one continuous mass of cytoplasm with 

 thousands of nuclei scattered through it. The 

 nuclei may be relatively fixed in position, or they 

 may move about freely with the streaming cyto- 

 plasm. In a number of coenocytic plants, moreover, the spores and 

 gametes themselves are coenocytic ("coenogametes" of Mucor and 

 Albugo hliti; zoospores of Vaucheria). 



8R. G. Sharp (1914), Yocum (1918), C. V. Taylor (1920), Rees (1921, 19226, 

 1931), McDonald (1922), Kofoid and Swezy (1922, 1923). 



^ Certain writers, notably Dobell (1911a), hold that the protozoan body is "non- 

 cellular" rather than "unicellular," and restrict the term "cell" to the integral parts 

 into which a "multicellular" organism is subdivided. Whatever may be thought of 

 the practicability of this use of terms, its theoretical implications are worthy of 

 attention. Although agreeing with Dobell that the protozoan body and the ordinary 

 tissue cell are not homologous, we have chosen to use "cell" more loosely as a term 

 of convenience for both of them, as well as for other units for which morphological 

 equivalence is not claimed. 



It is also frequently urged that "organelle" rather than "organ" should be used 

 for intracellular differentiations such as nuclei, plastids, and the neuromotor apparatus, 

 and that the latter term should be applied only to multicellular structures. We have 

 used "organ" in the more general sense in order to emphasize the fact that in all 

 cases the structures indicated are regional differentiations of protoplasm associated 

 with certain functions, to which the presence or absence of cell partitions is a sub- 

 ordinate feature. "Organelle" is, however, a useful term. 



Fig. 5. — Diplodinium 

 ecaudatum. M, mouth; 

 A'', neuromotor appara- 

 tus; Mic, micronucleus; 

 Mac, macronucleus; 

 C.V., one of the con- 

 tractile vacuoles; A, anal 

 canal; C, contractile re- 

 gion; S, skeletal plates. 

 {After R.G. Sharp, 1914.) 



