58 



THE MAMMALIAN EGG 



The most obvious modifications in shape of the vitellus are those 

 occurring in polar-body emission and in cleavage, but others are 

 seen also. Prior to polar-body formation, the surface becomes 

 elevated in the region that overlies the second maturation spindle; 

 the elevation may persist for hours or even days in the absence of 

 fertilization, and eventually subsides when the spindle breaks up. 

 A similar elevation develops at the site of attachment of the sperma- 

 tozoon and lasts for a short while after entry of the spermatozoon 

 into the vitellus. This reaction is analogous in some respects to the 

 outgrowth of the fertilization cone of many invertebrate eggs. 

 Unfertilized eggs often undergo fragmentation and in these circum- 

 stances the cytoplasmic masses may take on bizarre shapes, presum- 

 ably under the influence of disorganized cleavage forces. Some eggs 

 penetrated by X-irradiated spermatozoa have been observed to 

 share the same fate (Bruce and Austin, 1956) (Figs. 48 and 49). 



Figs. 48 and 49 



Mouse eggs cleaved after fertilization with X-irradiated spermatozoa. 

 Bruce and Austin, 1956.) 



420. (From 



Another form of movement evinced by the egg cytoplasm is a 

 constant steady streaming or 'boiling' motion which can best be 

 demonstrated by time-lapse photography. This is evidently the 

 same phenomenon as cytoplasmic streaming or 'cyclosis' which is 

 well known in other mammalian cells under conditions of tissue 

 culture but especially in plant cells. 



