EMBRYOLOGY 



397 



less tough, but still fairly dense, albumen, and this in turn is sur- 

 rounded by a thickish layer of more fluid albumen. These three 

 albuminous layers constitute the so-called white of the egg. The 

 albumen is confined in a tough bag, the shell membrane composed of 

 a double layer of fibrous matter, and the two layers separate at the 

 blunt end of the egg to include between them a small air chamber. 

 Finally, overlying the shell membrane is the calcareous shell itself, 



ae 



ad 



n 



FIG. 137. Hen's egg. -From Kellicott. 



A, entire "egg," modified from Marshall. B, vertical section through the vitellus or ovum 

 proper, showing the concentric layers of white and yellow yolk, a., air chamber; ac, chala- 

 ziferous layer of albumen ; ad, dense layer of albumen ; /, fluid layer of albumen ; b., blasto- 

 derm ; c,, chalaza ; /., latebra ; nl, neck of latebra ; P., nucleus of Pander ; pv, perivitelline 

 space ; smi, inner layer of shell membrane ; smo, outer layer of shell membrane v., vitellus 

 or " yolk " ; vm, vitelline membrane ; wy t layers of white yolk ; yy, layers of yellow yolk. 



which when dry is porous and allows of the passage of gases and 

 water vapour. All these structures are of the nature of tertiary egg 

 membranes. 



Maturation in the fowl's egg does not take place until just after 

 it has entered the oviduct and been fertilised. With the entrance of 

 the sperm the first polar body is extruded and immediately after 

 that the second is given off. Polyspermy appears to ba the rule, 

 but although a number of spermatozoa enter the egg, only one takes 



