62 THE MAMMALIAN EGG 



dna occurs during the interphase before each cleavage of the egg, 

 and the extra dna must presumably be synthesized from cyto- 

 plasmic substrate. Dalcq maintains that mucopolysaccharide and 

 'plasmalogen' (possibly an acetalphosphatide), the concentrations of 

 which have been found to fall immediately after mitosis and build 

 up again during interphase, are precursors of the dna. Indeed, it 

 is felt that the accumulation of these precursor substances to a 

 threshold level might initiate the new division. The mucopoly- 

 saccharide is located in groups of mitochondria that occupy, in 

 4-cell eggs and onwards, the peripheral parts of the blastomeres 

 destined to form the trophoblast, and its concentration increases as 

 this structure develops. Plasmalogen, on the other hand, is found 

 in the hyaloplasm. Both mucopolysaccharide and plasmalogen are 

 believed to originate in the nucleoli (which were shown often to 

 have metachromatic inclusions) and to pass into the cytoplasm when 

 nucleoli press up against the nuclear membrane. It is suggested, too, 

 that smaller nucleoli sometimes escape in toto into the cytoplasm. 

 In these ways, the cytoplasm is thought to be activated by sub- 

 stances that have derived from the chromosomes through the inter- 

 mediation of the nucleoli. 



Dalcq's theory is reminiscent in some respects of Kremer's (1924) 

 suggestion that substances originating in the cytoplasm pass into the 

 nucleus where they become specifically modified under the in- 

 fluence of genes, are stored in the nucleoli and eventually pass back 

 into the cytoplasm, within extruded nucleoli, as carriers of hereditary 

 characters. The idea, in general terms, seems reasonable enough, 

 though the transfer of nucleoli as such, or even of less organized 

 material, directly from the nucleus to the cytoplasm is inconsistent 

 with current views. It would be more acceptable to maintain that 

 the influence is indirect, a new substance being elaborated on the 

 cytoplasmic side of the nuclear membrane, but controlled in its 

 properties by gcnically determined agents within the nucleus. 

 There is a good deal of evidence that cytoplasmic rna is syn- 

 thesized under these conditions (see Brachet, 1957). In this connec- 

 tion, it is of special interest that in one species, the Chinese hamster, 

 there arc distinctive sacculations about the pronuclei (Fig. 52) and 

 cleavage nuclei which might well be associated with processes of 

 synthesis at the nuclear membrane. 



Dalcq further postulates that, as the rna concentration increases 

 in the inner cell mass of the implanted blastocyst, small granules 



