DEVELOPMENT 277 



talium, and a primitive Chordate, Styela. The egg of the latter 

 shows at the first division five clearly differentiated cytoplasmic 

 regions. For the sake of simplicity these may be described as 

 white, 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 white region 

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

 derm; while the cells with light or dark yellow form mesodermal 

 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 substances which possibly themselves directly, but 

 probably 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 pre- 

 delineated in bold lines, though not actually preformed, in the egg. 

 (Fig. 178.) 



Passing now to the second type, represented, for example, by the 

 eggs of some Sea Urchins, the results which we obtain seem to be 

 diametrically opposite. Although more or less clearly differentiated 

 cytoplasmic regions appear to exist, frequently the removal of a 

 part of the egg before division, or the separation of the cells at 

 the two-cell stage, and sometimes even at the four-cell stage, has no 

 permanent effect on the structural integrity of the developing 

 embryo. Each of the cells 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 four-cell stage, a single cell which nor- 

 mally forms, let us say, one-fourth of the embryo, if isolated with 

 one other cell, may form one-half of a normal embryo and, if isolated 

 with two other cells may form one-third. And apparently the same 

 phenomenon occurs in the case of human identical twins. The egg 

 becomes separated into two parts during early development and re- 

 sults in two individuals with identical hereditary basis. Indeed iden- 

 tical quintuplets, all from one zygote, have become famous. In all 

 such cases one may ask, what has become of the egg organization? 



At first glance the behavior of these two classes of eggs seems 

 to afford results which are irreconcilable — the former supporting 

 the doctrine of preformation in a refined form, and the latter its 



