286 Vertebrate Embryology 



of the sexual cells, ova and sperm, of man, 

 since it is with the development of man that 

 we shall be chiefly concerned. 



It has already been said (page 92) that the 

 variation in the size of ova is due mainly to 

 the amount of food yolk. In the human ovum 

 the food yolk is in very small quantity, so that 

 the cell is extremely small, about ^ mm. in di- 

 ameter ; while the ovum of the chick is 25-30 

 mm. in diameter. 



The human ovum (Fig. 85) consists of a 

 spherical cell, the ovum proper or vitellus, sur- 

 rounded by the zona pellucida and the corona 

 radiata. In the ovum proper there is a large, 

 spherical nucleus containing a distinct nucleo- 

 lus. According to Minot, the yolk is chiefly 

 collected near the centre of this part of the 

 ovum, leaving a protoplasmic, yolk-free, per- 

 ipheral zone. 



Between the vitellus and the surrounding 

 zona pellucida is a narrow perivitelline space 

 in which the vitellus is said to rotate freely, 

 in the living condition. The zona pellucida 

 is a clear area around the ovum proper ; it 

 shows radial striations which Minot thinks 

 may be due to radially arranged canals. The 

 outermost region of the ovum is the corona 



