CLEAVAGE AND DIFFERENTIATION 



and becomes therefore a definite characteristic not only 

 for cleavage but also for the whole course of develop- 

 ment. The body of a newly hatched chick contains 

 nuclear matter far in excess of that in the uncleaved 

 egg which gave rise to the animal. Indeed, as long as the 

 chick lives and is capable of producing germ-cells it elab- 

 orates nuclear substance. Moreover, every organism that 

 regenerates lost parts, every one whose tissues exhibit 

 pathological growths, as tumors, builds nuclear substance. 

 Hence, the building up of the chemical constituents of the 

 nucleus represents a basic property of protoplasm as a self- 

 reproducing system. In other words, as protein-synthesis 

 represents that fundamental chemical characteristic of 

 living matter that distinguishes it from non-living, so 

 nuclear synthesis (in part a protein-synthesis also) stands 

 as one of the primary chemical activities of the self-regula- 

 tion and the self-differentiation exhibited as special attributes 

 of cells capable of reproducing themselves. The course 

 of development is marked by syntheses. By these arise 

 secretions as those of the thyroid, the pancreas, etc., and 

 such compounds as haemoglobin, the iron-containing respir- 

 atory pigment of the blood, as found in the embryos of 

 animals which possess these substances. By synthesis also 

 nuclei arise. 



The question now Is: out of what are the nuclei synthe- 

 sized .^ With the progressive increase in amount of nuclei 

 runs during the period of cleavage a decrease in that of the 

 cytoplasm without any change in the amount of the yolk 

 and oil. This fact, holding for all animal eggs, is especially 

 clearly shown in totally cleaving eggs. In the egg of Nereis, 

 for example, I have followed very exactly the composition 

 of the blastomeres throughout cleavage and can state that 

 in the blastula-stage the total amount of cytoplasm Is 

 less than that in the egg at first cleavage, whereas the oil 

 and yolk do not change in amount. The clearest evidence 



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