256 FOUNDATIONS OF BIOLOGY 



be described as hyaline, light and dark gray, and light and 

 dark yellow. As cleavage proceeds, these substances are 

 distributed with great regularity to definite cell groups, 

 which in turn form special organs or organ systems of the 

 animal. Thus cells which receive the hyaline region form 

 the ectoderm; those which receive the dark gray, the endo- 

 derm; while the cells with light or dark yellow form meso- 

 dermal structures, 'and so on. And further, the experimental 

 removal of a cell or cell group in which a certain substance 

 is segregated results in an embryo deficient in the very 

 structures which this normally forms. In other words, the 

 egg cytoplasm seems to be a mosaic of 'organ-forming sub- 

 stances,' which either themselves directly, or through more 

 fundamental conditions of which they are but the visible 

 expression, have a causal relation to definite adult structures. 

 Just in so far as this is true, the adult is predelineated in bold 

 lines, though not actually preformed, in the egg. (Fig. 132.) 

 Passing now to the second type, represented by the eggs of 

 Amphioxus and the Sea Urchins, the results which we obtain 

 seem to be diametrically opposite. Although in the egg 

 of the Sea Urchin more or less clearly differentiated cyto- 

 plasmic regions appear to exist, the removal of a part of the 

 egg before division, or of one or more cells during cleavage, 

 blastula, or gastrula stages, has no permanent effect on the 

 structural integrity of the developing embryo. Experi- 

 ments show that each of the cells, even as late as the sixteen- 

 cell stage, has the power to develop into an embryo complete 

 in every respect, but smaller than the normal. Or, to put it 

 another way: at the sixteen-cell stage, a single cell which 

 normally forms, let us say, one-sixteenth of the embryo, if 

 isolated with two other cells, will form one third of a normal 

 embryo; if isolated with three other cells, will form one 

 quarter; and so on. What now has become of the egg 



