72 THE MAMMALIAN EGG 



The perforatorium probably plays a role in the penetration of the 

 spermatozoon through the zona pellucida and perhaps the vitelline 

 membrane. 



Fig. 62 

 Rat sperm tails, (a) lying in the cytoplasm of a 2-cell egg, 

 (b) suspended in the surrounding medium after an egg has been 

 broken. The component fibrils are becoming separated. A 

 'smoke ring' is visible around the tail shown in (a). X 900. 



Mechanism of Cell Division 



Cytoplasmic division is an almost universal characteristic of cells 

 and as a general rule it immediately succeeds nuclear division. The 

 cell elongates and the surface around the lesser circumference dips 

 inwards towards the equator of the spindle. The equatorial plane is 

 often marked by the presence of the intermediary body (Fig. 55), 

 which consists of basophilic granules considered to be rna left 

 behind by the chromosomes after anaphase separation. The con- 

 striction continues until the cell is divided into two daughter cells 

 within each of which a resting nucleus is reconstituted. The plane 

 of cleavage passes to one side of the intermediary body and not 

 through it, and the residue of the spindle bearing this structure can 

 often be discerned shortly after cleavage (see, for example, Fig. 24 

 of De Robertis, Nowinski and Saez, 1954). 



