22 INTRODUCTION TO CYTOLOGY 



resolution of one persistent whole into newly formed metabolic units: 

 it should be thought of not as primarily a multiplication and cooperation 

 of cells but rather as the differentiation of growing protoplasm. 



The real unity is that of the entire organism and, as long as its cells remain in 

 continuity, they are to be regarded not as morphological individuals but as 

 specialized centers of action into which the living body resolves itself and by 

 means of which the physiological division of labor is effected (Wilson, 1893). 



The phylogenetic implication of the organismal theory is that if 

 multicellular organisms have arisen from unicellular forms, the process 

 may have been the same as that seen in ontogeny: the differentiation 

 and subdivision of a continuous growing mass of nucleated protoplasm 

 into a system of uninucleate cells. If the septation coincided with 

 nuclear division from the first, there was a direct transition to forms with 

 several cells. If the two processes did not so coincide, ccenocytic or 

 "plasmodial" types arose, some of which persisted as our ccenocytic 

 organisms while others developed internal walls and became our multi- 

 cellular forms. The ability of isolated tissue cells to live independently 

 does not prove that independent cells have combined to form the multi- 

 cellular body but only that such parts can still carry on essential proto- 

 plasmic functions. The phenomenon could as well be taken to prove the 

 derived nature of Protista. The body is not an aggregation of elementary 

 organisms, but a single organism which has evolved a multicellular structure. 

 The cell colonies in certain green algae-" and the remarkable polyp colonies 

 in the Siphonophora (see J. S. Huxley, 1912) indicate the dependence of 

 the part upon the whole even in a group formed by aggregation, and they 

 show the possibility of an evolution by the combination of individuals; 

 but it is not at all clear that they afford the key to the evolution of organ- 

 isms in general. The protozoon is more properly compared with the whole 

 man ; both are organisms which have differentiated a series of specialized 

 internal regions or organs, the one without cellular subdivision or increase 

 in size and the other with them. 



Some of the cytological facts underlying the organismal theory may be 

 briefly cited as follows. 



Many algse and other organisms, as already noted, develop bodies of 

 considerable size and with definitely differentiated form and structure 

 without any cellular subdivision of the multinucleate protoplasm. In 

 some of these {e.g., Bryopsis; Noll, 1903) the tips of the plant grow steadily 

 forward as wholes, in spite of the fact that the cytoplasm and the nuclei 

 it carries are constantly being changed by protoplasmic streaming. 

 Interpreting the entire plant as a huge cell accomplishing the development 

 is here equivalent to admission that it is the whole which dominates the 

 "centers of activity" about the nuclei. 



2" Bock (1926) describes plasmodesms in Pandorina, Eudorina, and Gonium. 



