STRUCTURE AND FUNCTION IN MAMMALIAN EGGS 61 



plasm of rat oocytes and fertilized eggs, distributed throughout the 

 matrix of the cytoplasm and without any obvious association with 

 other cytoplasmic structures. There were more particles in fertilized 

 and developing eggs than in oocytes. It seems probable that the 

 rna in these particles is responsible for the ultra-violet absorption 

 reported by Austin and Braden (1953c) in the hyaloplasm of eggs; 

 the particles are certainly much too small to correspond to the red 

 fluorescent granules observed by fluorescence microscopy. 



The extensive observations of A. M. Dalcq and his colleagues 

 (see Dalcq, 1955a, 1956, 1957; Borghese, 1957) have led to different 

 conclusions. In addition, their findings have been built into the 

 theory that cytoplasmic characteristics confer a bilateral symmetry 

 on the oocyte and the egg during fertilization, and later serve to 

 distinguish those regions of 2- and 4-cell eggs, and those blastomeres 

 of 8-cell eggs, that are to become either the inner cell mass or the 

 trophoblast of the blastocyst. Symmetry of the oocyte is held to be 

 due to the presence of a more 'condensed' form of cytoplasm, 

 containing more and larger mitochondrial granules, at the animal 

 pole and on one side of the animal-vegetal axis ; this region contains 

 more rna as indicated by the basophilia detectable by pyronine 

 staining, before but not after treatment with ribonuclease. On the 

 other side of the cell, the cytoplasm is somewhat vacuolated, con- 

 tains fewer granules, and, in animals such as the guinea-pig, is 

 distinguished by the presence of numerous fat globules. The planes 

 of cleavage are not clearly related to the plane of symmetry, but, 

 when the 8-cell stage is reached, four of the blastomeres are found 

 to be relatively larger than the others. The larger blastomeres 

 contain the more vacuolated cytoplasm and these are the ones 

 destined to constitute the trophoblast by increase in size with low 

 mitotic frequency. The smaller blastomeres, richer in rna, in- 

 crease further their content of nucleic acid as they rapidly multiply 

 to form the inner cell mass. Thus, both the form and distribution 

 of the rna bodies define the future development of parts of the 

 egg and early embryo. In both types of cell, the rna is described 

 as being associated with the larger mitochondrial granules which 

 are distributed in the outer regions of the cells. Some rna, how- 

 ever, accompanies the finer granules which gather near the nuclei. 

 No rna, apparently, is identified in the hyaloplasm. 



For Dalcq, the distribution of rna is only part of the story. 

 As he and Pasteels (1955) have shown, doubling of the nuclear 



