CLEAVAGE AND DIFFERENTIATION 



The positions especially of the oil and the yolk in the 

 egg-plasma change after fertilization, as comparisons of the 

 unfertilized with the fertilized eggs of either Nereis or 

 Chaetopterus reveal. In the egg of Arhacia as of any other 

 echinid the shift in oil and yolk is not so well marked. 

 That is, in eggs with determinate cleavage there is a more 

 clear-cut change of the cytoplasmic inclusions to new posi- 

 tions than in indeterminately cleaving eggs. The progres- 

 sive differentiation from the single cell, the egg, to a complex 

 organism, the embryo, was once thought of as due to the 

 distribution of the visible materials in the cytoplasm: 

 embryo-formation was ascribed to the segregation of these 

 fat- and fat-containing materials which were therefore 

 denominated organ-forming substances. To-day, we can 

 appreciate the naivete of this theory, wondering how we 

 could have given it such serious consideration. 



Eggs differ^ with respect to content of cytoplasmic inclu- 

 sions. This is especially true of yolk. Indeed, eggs are 

 sometimes classified on the basis of yolk-content. Once 

 we spoke of yolk-less (alecithal) eggs; now we know that 

 all eggs, even the human, contain yolk. Thus it is better to 

 classify eggs not as yolk-free and yolk-rich, but rather as 

 telolecithal (yolk in one part), centrolecithal (yolk in the 

 center) and homolecithal (yolk evenly distributed). \ oik 

 also differs physico-chemically in different eggs; and I have 

 some evidence which indicates that it differs in a given egg 

 before and after fertilization. What is true of yolk, the 

 food material for the embryonic organism, is true of mito- 

 chondria, pigment-granules, etc.: they vary as to amount, 

 distribution and physico-chemical make-up. Whilst all 

 of these cytoplasmic inclusions of which I here speak differ 



^ The reader will have noted thai I ojten emphasize the differences 

 in animal eggs. In this my general purpose is to sift from the 

 differences that which is similar and equivalent and thus to order 

 and to define the biological problems. 



