THE LABILE NATURE OF ORGANIZATION 35 



were fixed in certain positions within the egg, the expected result from half 

 an egg would be half of an embryo. However, since the argument has validity, 

 let us proceed to some other experiments which prove that the egg is labile. 



For this purpose let us refer to the original experiment of cutting the 

 egg in two through the equator. We can experiment further with the animal 

 halves, which contain the factors for ectoderm only. Some of the animal 

 halves can be raised in normal sea water, while others are raised in lithium 

 sea water — that is, sea water to which lithium chloride is added. Under these 

 conditions the animal halves in normal sea water form permanent blastulae, 

 which do not undergo any differentiation into mesoderm or endoderm. 

 However, the lithium-treated animal halves will form larvae with a gut, 

 skeleton, and ectoderm (Fig. 15). Therefore, simply by the addition of 

 lithium — and other compounds will also bring about this change — the 

 region of the egg which normally gives rise to ectoderm can be converted 

 into some endoderm. Ectodermal factors plus lithium may give rise to some 

 mesoderm and endoderm. 



Lithium chloride may, of course, produce many physical and chemical 

 changes in the protoplasm of the animal half. We know of one change 

 which does occur in lithium-treated eggs — that is, a decrease in the rate of 

 oxidation. Therefore it may be that the animal half is respiring at too high 

 a rate to develop into mesoderm and endoderm, and that the action of 

 lithium is to cut down this high respiration rate to the rate that is normal 

 for mesoderm and endoderm. 



Conversely, the mesodermal and the endodermal regions may be con- 

 verted into ectoderm. If we begin with the unfertilized egg and treat it with 

 potassium sulfocyanide in sea water, the egg when fertilized develops into 

 the same type and structure that develops from the animal half — a perma- 

 nent blastula. No mesoderm and no endoderm appear. Therefore potassium 

 sulfocyanide must have brought about some change in the vegetal hemisphere 

 so that the material normally destined to form mesoderm and endoderm 

 became converted into ectoderm. Thus there are both kinds of change: 



Li 



I 

 "ectoderm'' < * "endoderm" 



t 



KSCN 



