78 DIFFERENTIATION III 



be formed by a part of the germ which is not ordinarily 

 devoted to that end. We must therefore assume provisionally 

 some such disposition of the organ-forming materials in the 

 germ as that suggested by Boveri in his 'stratification' 

 hypothesis. According to this conception the substances in 

 question are distributed through the cytoplasm, but with con- 

 centrations decreasing in opposite directions, which directions 

 are, in polar eggs, along the axis. Thus a substance on which 

 the formation of the ectoderm depends is assumed to be most 

 concentrated at the animal pole, less in the equator, and least 

 at the vegetative pole, while there is a similar decrease of 

 endoderm-forming substance in the reverse direction along 

 the axis. Both animal and vegetative cells therefore possess 

 the requisite materials for developing these structures, though 

 not in the same proportions as in the entire ovum, nor as one 

 another. Hence the differences in their capacities. 



Such a stratification can of course in some cases be actually 

 observed, for instance in the Amphibian egg, where yolk and 

 cytoplasm are graded in opposite directions in this way, and 

 even where not easily visible may be made apparent by the 

 use of the centrifuge, since the several constituents of the 

 cytoplasm are frequently of different specific gravities. 



The effect of separating these constituents in this way 

 has now been investigated on several eggs and much light 

 thereby thrown on the significance of each one in develop- 

 ment. 



Ever since Born's cytological examination of the Frog's egg 

 forcibly inverted in Pfliiger's original experiment, it has been 

 known that the heavy yolk-granules sank in the viscid cyto- 

 plasm while the lighter plasma rose to replace them, so con- 

 ferring a new polarity upon the egg ; and ever since Hertwig's 

 experiment it has been known that by this use of the centrifuge 

 the yolk could be driven much more rapidly to one side of the 

 egg than under the influence of gravity alone. Such eggs 

 frequently develop abnormally, with rneroblastic segmenta- 

 tion, spina bifida of the embryo, and so on. The development 

 of such centrifuged eggs has been more recently studied by 

 Konopacka. The eggs, placed on slides in water, are centri- 



