CELLS AND TISSUES 23 



The same general result may be achieved with or without cell-forma- 

 tion, as in the bodies of Vaucheria, Cladophora, and Stigeoclonium, which 

 show essentially the same degree of elaboration although the first is 

 ccenocytic, the second semicoenocytic, and the third strictly cellular. 

 Eggs of Chcelopterus have been observed to differentiate into swimming 

 larvae even when the usual cytoplasmic cleavages are suppressed by add- 

 ing some KCl to the sea water (F. R. Lillie, 1902, 1906). It has been 

 shown in a number of cases that alterations in the position of successive 

 cleavage furrows do not disturb the normal course of development and 

 differentiation. In the frog, for example, "normal development does not 

 depend upon a specific number and succession of cleavages in definite 

 positions but rather upon an egg pattern which may be cut up by the 

 cleavage furrows in various ways without destroying the pattern or the 

 normal results of development" (Conklin, 1924). Spemann (1918 et 

 seq.) has shown that in very young amphibian embryos groups of cells can 

 be cut out and reversed in position or transplanted into other regions, 

 whereupon they develop as parts of the organs normally differentiating in 

 those regions and not as those of the regions from which they came. 

 Thus, as F. R. Lillie (1902) concluded for Chcetopterus, "the process of 

 cell-division, as such, is necessary neither to growth, differentiation, nor 

 the earliest correlations; but it is accessory, in Metazoa, to all three as a 

 localizing factor, often from the earliest stages. " 



It was to the same conclusion that W. Hofmeister, Sachs, de Bary, and 

 other botanists were led many years ago through detailed studies of cell- 

 formation in the growing regions of plants. They found the growth of the 

 organ as a whole to be the primary matter, the position of the cell walls 

 within it being secondarily determined by the physical forces acting within 

 the growing mass. "The formation of new cells in the vegetative point is 

 accordingly a function of the general growth, not its cause" (Hofmeister, 

 1867). This view has been upheld by many researches on the mechanics 

 of growth and form (see Thompson, 1917) and by the recent studies of 

 Sinnott (1930) on the relation of the dimensions of cells to the size and 

 form of the organ they constitute. 



Many organisms which are cellular throughout the greater portion of 

 the life cycle pass through a ccenocytic phase, often at a critical stage in 

 the cycle, and in this phase as elsewhere growth and differentiation con- 

 tinue. In the young embryos of Agathis (Fig. 13) and other gymnosperms 

 the characteristic mode of development is indicated by the positions 

 taken up by the nuclei during the ccenocytic stage, the position of the 

 subsequently formed cell partitions being determined by differentiations 

 occurring in this stage. Similarly, there are among animals cases in 

 which the embryo first passes through a free-nucleate stage, subdivision 

 into cells occurring after differentiation, particularly that of the germ 

 region, is well on its way. That differentiation in such cases is a function 

 of the protoplasmic mass as a whole is indicated by the fact that any 



