THE BIOLOGY OF THE CELL SURFACE 



that nuclei arise out of the cytoplasm is furnished by the 

 development of those clear hyaline pieces of protoplasm 

 already referred to, for in the cleavage-process of such frag- 

 ments also the nuclear matter increases and the cytoplasm 

 decreases. It has been shown that the nuclei in yolk- 

 free blastomeres are larger than those in yolk-rich blasto- 

 meres and that the size of the nuclei is proportional to the 

 hyaline content of a blastomere rather than to the total, 

 encompassing the inclusions.^ These facts prove that in 

 totally cleaving eggs during cleavage the nuclei are synthe- 

 sized from the cytoplasm. In partially cleaving eggs as well 

 the yolk takes no part in development during cleavage. 

 When, as happens in these eggs, at the end of cleavage new 

 cells are added to the embryonic area from the yolk region, 

 the yolk, as the evidence indicates, is transformed into cyto- 

 plasm and does not go directly into the nucleus. 



The increased nuclear content at the end of cleavage 

 therefore can only mean that the nuclei are elaborated out 

 of nuclear stuffs or their precursors; it can not mean an 

 automatic activity of the nucleus capable of building nuclei 

 out of nothing. Nuclei are built up from the cytoplasm. 

 So too their constituents, the chromosomes. 



In preparation for fertilization both egg and sperm-cell 

 go through a process by which the number of chromosomes 

 characteristic for the species is halved; at fertilization this 

 number is restored, half from the father, half from the 

 mother. This is the strongest evidence in favor of the 

 chromosomes as the means by which the off-spring inherits 

 qualities from both parents. The number of chromosomes 

 of the fertilized egg is the somatic or so-called diploid 

 number characteristic for the species. During development 

 this number remains constant. Every somatic cell of the 

 adult organism no matter how complex contains in its 



^ Conklin, igi2. 



310 



