CLEAVAGE AND DIFFERENTIATION 



of this egg as a means of orientation in experiments. But 

 the occurrence of this band is inconstant; not every speci- 

 men of Strongylocentrotus gives eggs containing it. There 

 is no virtue in colored particles as such suspended in the 

 cytoplasm for differentiation, though they may be useful 

 indicators in other respects. 



Another cogent argument against the theory that the 

 visible cytoplasmic inclusions are organ-forming substances 

 lies in the fact that cleavage in many eggs may ensue for 

 several cell generations apart from the underlying yolk and 

 oil; this is true of all eggs with discoidal cleavage. In them 

 the cytoplasm is at first a transparent and apparently 

 homogeneous sheath and later a disc below the vitelline 

 membrane. Cleavage is confined to this inclusion-free 

 cytoplasm. When in later stages of development — after 

 the differentiation occurring during cleavage has taken 

 place — the embryo grows at the expense of the yolk-laden 

 region of the egg, the yolk is transformed into clear cyto- 

 plasm. In other words, in these as in all animal eggs, 

 yolk and oil play no direct part in differentiation.^ 



Let us recall another fact: many living eggs under the 

 microscope appear to be transparent. Rigorously to adhere 

 to the theory of visible organ-forming substances is to deny 

 organ-formation in transparent eggs. By ascribing a 

 primary role to the visible inclusions in the cytoplasm, we 

 have had the tendency to reckon the flood by its burden, 

 taking the traffic for the stream. 



Finally, certain experimental results set up an unsur- 

 mountable obstacle to acceptance of the theory of organ- 

 forming substances as an explanation for differentiation. 



Above I have pointed out that due to the ectoplasmic 

 changes caused by fertilization, eggs exhibit cytoplasmic 



^ In trematode eggs exists the strongest separation between egg and 

 yolk, because the egg alone is a product of the ovary; the yolk is 

 produced by accessory glands and spun around the egg. 



305 



