THE BIOLOGY OF THE CELL SURFACE 



from each other, they are, so far as I know, wholly or In part 

 composed of lipin (fat or fat-like substance). Though there 

 Is no reason against the postulate that the organ-forming 

 substances are composed of fats, nevertheless, I believe 

 that we are on unsafe ground in attempting to relate the 

 formation of the many various organs to fat-containing 

 compounds. 



Sometimes the cytoplasmic Inclusions show diiferential 

 distribution to very definite blastomeres. Consider the 

 egg of a low form of chordate, for example, that of the tuni- 

 cate, Cynthia. One substance (yellow) maintains a defi- 

 nite distribution to given blastomeres. This is an egg 

 with determinate cleavage. From the blastomeres contain- 

 ing the yellow substance always in the normal Qg^ muscle 

 and mesenchyme develop. Apparently, therefore, here Is 

 a very clear case of an organ-forming substance definitely 

 localized In the Qgg before first cleavage. But experimental 

 findings do not permit such a conclusion. To these I shall 

 later turn. 



Other eggs with determinate cleavage for the most part 

 do not exhibit prelocalizatlon so clearly as that of Cynthia^ 

 though their cytoplasmic Inclusions shift from their original 

 positions before fertilization to new ones after fertilization 

 and at first cleavage; thus the yolk and oil come to lie in 

 cells which are destined to form the gut. This Is not evi- 

 dence that oil and yolk "determine" gut-formation. Also, 

 there are many other eggs that do not show any special or 

 constant disposition of the cytoplasmic inclusions. 



On other grounds the theory that the visible cytoplas- 

 mic Inclusions are organ-forming substances Is untenable. 



In the first place, I call attention to the egg of Strongylo- 

 centrotus lividns, a sea-urchin commonly found in the 

 Mediterranean, which may exhibit a beautiful superficial 

 band of orange pigment below the equator In the hemisphere 

 opposite the animal pole. Boverl used the pigment band 



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