100 



THE CELL AND PROTOPLASM 



cytoplasm to that of the nucleus is in the 

 order of magnitude of 1 to 25 to 1 to 50. 

 It is also obvious to students of embryonic 

 tissues that the rate of metabolism and 

 stage of growth affect this ratio. Pole cells 

 and primordial germ cells have proverbi- 

 ally larger nuclei than neighboring cells. 

 The shape of the cell and that of the nu- 

 cleus are also correlated. Both cell and 

 nucleus elongate in the same axis. Branch- 

 ing Suctoria have thread-like branching 

 nuclei in the axes of the cylindrical columns 

 which form the hydroid-like colony. 



It is also obvious that the location of the 

 nucleus is correlated with the functional 

 activity of the cell rather than with the 

 spatial relations only. In a yolk-forming 

 oocyte the nucleus lies near the surface 

 next to the blood supply, not in the center 

 of the cell. In a budding filamentous alga 

 the nucleus lies at the point where the bud 

 is forming. 



These basic and universal facts of size, 

 spatial relations, and relations to metabolic 

 activity, so evident in most cells, are in- 

 dicative of an equally general functional 

 principle, namely, that the activities of the 

 cell can only be carried on within certain 

 rather small limits of size. One of the many 

 factors involved in this by no means simple 

 matter is undoubtedly the ratio of the sur- 

 face of the nucleus to the volume of the 

 cytoplasm. In this ratio the nuclear mem- 

 brane and the peripheral chromatin are 

 directly involved. Peripheral chromatin 

 provides for intimate interaction of the 

 contrasted chromatin and cytoplasm across 

 the nuclear boundary. It is wont to be 

 more abundant in rapidly growing tropho- 

 zoites than in encysted stages. In premi- 

 totic phases of Amoeba the chromosomes are 

 lined up against the nuclear membranes 

 and split while in this position. In leuco- 

 cytes whose activity is great and whose life 

 may be brief the relatively large amount of 

 peripheral chromatin is noteworthy. 



Since the surface of the nucleus increases 

 only in the ratio of the square while its 

 volumes of cytoplasm and nucleus increase 

 in the ratio of the cube it is mathematically 

 evident that an increase in the size of the 

 cell would decrease the relative contact of 



nucleus and cytoplasm. The volumetric 

 units of cytoplasm increase at a higher rate 

 than the units of surface of the nuclear 

 membrane and thus the efBciency of the 

 nucleus with its contained chromatin and 

 genes drops rapidly with the increase in the 

 volume of the cell. The Protista are small 

 and tissue cells are limited to small dimen- 

 sions simply because the activities of living 

 demand, for efficient action, certain sur- 

 face-volume relations of nuclear membrane 

 to the enveloping cytoplasm in which all 

 structural differentiation and much func- 

 tional activity occur. It is equally obvious 

 that other factors enter into the functional 

 activities of the cell, such as temperature, 

 light, food, age, location, contacts, stimuli, 

 and past history, all of which may singly 

 or in various combinations accelerate or 

 diminish the metabolic rate and other 

 activities of living. 



Since the living substance is by nature 

 acquisitive and the primitive one-celled or- 

 ganism increases its volume by growth, 

 there inevitably follows a lowering of effi- 

 ciency which is, however, restored by cell 

 division and a resumption of the optimum 

 surface-volume relations. It is therefore 

 of basic significance that asexual reproduc- 

 tion by fission is prevalent in the Protista, 

 among bacteria, and in primitive Protozoa 

 and Protophyta. In fact, it is the only 

 method of reproduction known among 

 these primitive forms. It is only among 

 the higher Protozoa and Protophyta that 

 sexual reproduction, based on proved meio- 

 sis, has as yet been detected. 



If, then, in the course of the evolutionary 

 process, the organism is to acquire in- 

 creased energy and to add to its fourth di- 

 mension of time, it must have recourse to 

 an increase in the number of its working 

 mechanisms, the cells. This increase also 

 affords space, volume, and time for diversi- 

 fication in function among the units within 

 the organism. 



We, therefore, turn naturally to the Pro- 

 tista for concrete evidence of the method 

 by which the primitive organisms have met 

 this problem of their evolution from the 

 unicellular to the multicellular state. Be- 

 cause of a greater familiarity with the Pro- 



