56 THE MAMMALIAN EGG 



beginning of a differentiation of the endoplasmic reticulum. Very 

 numerous small dense particles, identical with the rna particles 

 responsible for basophilia, were distributed throughout the cyto- 

 plasm and were more common in eggs undergoing fertilization and 

 early cleavage than in oocytes. The bulk of the vitellus was finely 

 granular in appearance and more or less uniform in texture; this 

 material was considered to be deutoplasmic in nature. Scattered 

 throughout, however, were numerous irregular masses of a more 

 densely granular nature, often connected by bridges or trabeculae 

 and containing many mitochondria and other small bodies; this 

 material probably represented the active cytoplasm. Among the 

 other small bodies just mentioned, there were many examples of 

 an unusual type of structure — a vesicle containing many small 

 vesicles. This was termed a 'multivesicular body' or 'vesicular 

 conglomerate' ; these bodies increased in number during maturation 

 and fertilization, and they were believed to break down in the later 

 stages, liberating their content of smaller vesicles. Odor (i960) 

 confirms the increased occurrence in the later stages of oocyte 

 growth. (Similar structures have been seen in glomerular epithelial 

 cells by Yamada, 1955, and in spider oocytes by Sotelo and Trujillo- 

 Cenoz, 1957.) In the ovarian oocyte, the surface of the vitellus was 

 found to be thrown up into microvilli which project a short distance 

 into the zona pellucida. Processes from the overlying follicle cells 

 also penetrate the zona and to a greater extent, often passing com- 

 pletely through, but no continuity appeared to be established 

 between the cytoplasm of follicle cells and oocyte (Fig. 47). 



The ultrastructure of the cytoplasm in tubal eggs of the golden 

 hamster appears to be similar in general to that described by Sotelo 

 and Porter for the rat. Here, too, the finer more homogeneous 

 material making up the bulk of the vitellus is liberally interspersed 

 with irregular groups of a coarser substance containing bodies 

 resembling mitochondria (Fig. 27). Multivesicular bodies were not 

 seen. 



Changes in size and form. Observations on the eggs of the rabbit 

 and the common laboratory rodents indicate that the size and shape 

 of the vitellus, in these eggs at least, is maintained dynamically and 

 not merely by physical conditions such as surface tension or cortical 

 rigidity. The vitellus can undergo a comparatively sudden reduction 

 in diameter, the contraction being associated with a release of fluid 

 into the perivitelline space. Contraction occurs most noticeably on 



