64 THE MAMMALIAN EGG 



to be more numerous in the central than in the peripheral regions, 

 and later become closely gathered about the first cleavage spindle. 

 In the 2-cell stage, the distribution is similar to that of the pronuclear 

 egg (Gresson, 1941, 1948). 



Mitochondria of a roughly spherical or oval form, but with the 

 characteristic internal cristae, have been described by Yamada, 

 Muta, Motomura and Koga (1957), Moricard (1958), Sotelo and 

 Porter (1959) and Odor (i960) in ultra-thin sections of mouse and 

 rat eggs, and their distribution corresponded broadly with that 

 observed by Gresson. Yamada et al. remarked on a feature that 

 seems peculiar to oocyte mitochondria, namely vacuolization. The 

 vacuoles vary in size, and are round or irregular in shape; they are 

 bounded by single membranes and appear less opaque than the 

 surrounding matrix. Bodies of the same size and shape as mito- 

 chondria and lying in the same pattern, are visible also in hamster 

 eggs, both penetrated and unpenetrated, but the absence of cristae 

 precludes their recognition as fully differentiated mitochondria 

 (Figs. 27 and 54). The arrangement of the red fluorescent granules 

 in rat eggs, evident after treatment with acridine orange, is also 

 similar to that of mitochondria (Figs. 15, 16, 25, 26, 35 and 36). 



Golgi material. In early oocytes, a strongly argentophilic and 

 osmiophilic structure, identified as the Golgi apparatus, is readily 

 demonstrable associated with the yolk nucleus at one side of the 

 germinal vesicle. As the oocyte grows, the Golgi material breaks 

 up, becoming distributed around the nucleus and later throughout 

 the cell, often in association with the groups of mitochondria. In 

 centrifuged oocytes of the mouse, granules of Golgi material fill a 

 broad band separate from that occupied by mitochondria (Gresson, 

 1940a). During fertilization and the first cleavage division, the Golgi 

 granules tend to gather about the pronuclei, particularly just before 

 syngamy, and also about the 2-cell nuclei (Nihoul, 1927; Gresson, 

 1948). 



By electron micrography of mouse and rat eggs, Yamada et a\. 

 (1957), Sotelo and Porter (1959) and Odor (i960) found a complex 

 structure containing paired membranes and a number of spherical 

 vacuoles which varied greatly in size; this was disposed close to one 

 pole of the nucleus in the early oocyte and was considered to consist 

 of Golgi material. In late oocytes, small groups of parallel mem- 

 branes were scattered chiefly through the peripheral cytoplasm. In 

 the cytoplasm near the arrays, there were numerous small vesicles 



