STRUCTURE AND FUNCTION IN MAMMALIAN EGGS 19 



Muta, Motomura and Koga, 1957; Sotelo and Porter, 1959; Odor, 

 i960). Scattered irregular aggregates of denser granular material 

 were observed within the nucleus, lying free and also in contact 

 with the nuclear membrane and the nucleolus. It was agreed, too, 

 that the nucleolar substance consists of closely-packed small dense 

 granules and bears no evidence of a limiting nucleolar membrane. 

 Descriptions of the general structure of the nucleolus varied, how- 

 ever. According to Yamada et ah, most of the nucleoli they saw in 

 mouse eggs were made up of a coarse irregular framework, the 

 meshes having ovoid profiles and being occupied by finer granular 

 material like the bulk of the nucleoplasm. The structure is strongly 

 reminiscent of the nucleoloneme as seen in oocytes of non-mammals 

 and in tissue cells (see De Robertis, Nowinski and Saez, 1954). In 

 addition, there was often found, attached to the nucleolus, an 

 irregular mass of lower density which also presented some indica- 

 tion of a network. Sometimes this body extended towards, and 

 even became attached to, the nuclear membrane. The authors 

 suggested that this represents the nucleolus-associated chromatin. 

 By contrast, Sotelo and Porter, who worked on rat eggs, reported 

 that oocyte nucleoli lack obvious organization, except for a broad 

 subdivision of nucleolar substance into a finely granular core 

 surrounded by a thick outer layer or wall of much denser con- 

 sistency. The wall substance resembled the material composing the 

 chromosomes that were found in sections of a secondary oocyte, 

 and it is possible that the thick wall may have represented the dna 

 shell referred to above. Differences in nucleolar structure are pro- 

 bably due to differences in the stage of oocyte development. Sotelo 

 (1959) described in the nuclei of rat primary oocytes the presence 

 of pairs of ribbon-like threads twisted around a thinner medial 

 element; often these structures appeared to be associated end-on to 

 the nuclear membrane as though attached to it. They evidently 

 represent the form taken by chromosomes in the oocyte nucleus. 



It has often been maintained that, in the oocytes of amphibia and 

 other non-mammalian forms, nucleoli pass bodily into the cyto- 

 plasm, possibly through a pinching-off of the nuclear membrane 

 (see Vincent, 1955, and Brachet, 1957). Migration is said to occur, 

 too, in mammalian oocytes (Makino, 1941) and in eggs undergoing 

 fertilization (Kremer, 1924, who also reviews the earlier literature; 

 Izquierdo, 1955; Dalcq, 1955a). Sotelo and Porter (1959) report 

 finding an object like a nucleolus in the cytoplasm by electron 



