86 



THE MAMMALIAN EGG 



Membranes and Investments 



Vitelline Membrane 



The egg cytoplasm, like that of other cells, is limited by a plasma 

 or permeability membrane. In mammalian eggs, the plasma mem- 

 brane is generally called the vitelline membrane, but it is not as well 

 developed as the vitelline membrane in the eggs of Sauropsida, nor 

 is it to be identified with the vitelline membrane of invertebrate 

 eggs, a structure that becomes modified after sperm entry and rises 

 from the egg surface as the fertilization membrane. Alone among 



the eggs of placental mammals, 

 the hamster egg has been said to 

 develop a fertilization membrane 

 (Graves, 1945; Venable, 1946), 

 but this could not be seen in 

 living eggs (Samuel and Hamil- 

 ton, 1942; Austin, i956d) and 

 there seems to be no evidence for 

 its existence in sections examined 

 by the electron microscope (Fig. 

 70). 



The vitelline membrane may 

 be considered to have essentially 

 the same structure and the same 

 properties of diffusion and active 

 transport as the plasma mem- 

 brane of tissue cells. (The struc- 

 ture and properties of the cell membrane have recently been 

 discussed by Fitton Jackson, 1961, and Weiss, 1961.) Osmotic 

 regulation in the vitellus is considered later as a feature of metabolism 

 (p. in). Active transport is probably involved in the fluid extrusion 

 associated with first-polar-body emission and with activation of the 



e gg (P- 56). 



As revealed by means of the electron microscope, the vitelline 

 membrane of the early oocyte is a smooth uncomplicated layer 

 against which the plasma membranes of the surrounding follicle 

 cells are closely applied. As the follicle develops, the vitelline mem- 

 brane becomes thrown up into numerous microvilli some of which 

 form interdigitations with the surface of the follicle cells or of 

 processes arising from them. With the formation and growth of 



Fig. 70 

 Electron micrograph of a penetrated 

 golden-hamster egg, showing part of the 

 sperm tail apparently enclosed within a 

 vesicle. X 14,000. 



