52 THE MAMMALIAN EGG 



of the embryo occurs, however, is there evidence of the cyto- 

 plasmic basophilia and the high nucleolar rna concentration, 

 which form integral parts of the Caspersson system (Alfert, 1950), 

 and it therefore seems unlikely that protein synthesis is a quantita- 

 tively important feature of metabolism in the embryo during 

 cleavage. Consistently, Greenwald and Everett (1959) have reported 

 that evidence for active protein synthesis, as inferred from uptake 

 of [ 35 S] methionine, was clearly shown by mouse ovarian eggs and 

 blastocysts, but not by embryos in the cleavage stages. Other 

 aspects of the nucleocytoplasmic relationship in processes of synthesis 

 are discussed later (p. 61). 



On the other hand, there is no reason to doubt that dna syn- 

 thesis takes place during cleavage. The mammalian egg lacks the 

 large cytoplasmic stores of dna that have been demonstrated in 

 sea-urchin and frog eggs (Zeuthen, 195 1; HorT-Jorgensen, 1954) 

 and the total nuclear dna is doubled at each stage of cleavage 

 (Dalcq and Pasteels, 1955). The increasingly large size of the 

 perinucleolar masses can be attributed simply to the physical result 

 of the packing of the same amount of material into progressively 

 diminishing nuclei. Despite this effect, the characteristic concentra- 

 tion of DNA-protein designated the 'sex chromatin' (Barr, Bertram 

 and Lindsay, 1950; Graham, 1954) does not become discernible in 

 cat, monkey and human embryos until the end of cleavage, that is 

 to say, at the time of implantation or shortly beforehand (Austin and 

 Amoroso, 1957; Park, 1957; Austin, 1961b) — approximately when 

 the size of embryonic cells has reached the size of an average tissue 

 cell. 



Cytoplasm 



Physical Features 



Yolk. Among the most characteristic features of the cytoplasm 

 of eggs is the presence of stored nutrient material (yolk or deuto- 

 plasm) and the manner in which it is distributed. On the basis of 

 the amount of yolk eggs contain, they can be classified into two 

 groups: those with much, the mcgalccithal eggs, and those with 

 little, the miolecithal eggs. This subdivision is somewhat arbitrary, 

 for there exists in the animal kingdom as a whole a continuous 

 series between the extreme forms. The mcgalccithal egg consists 

 essentially of a mass of yolk on the surface of which lies a small 

 cytoplasmic disc wherein the nuclear structures reside and which 



