14 THE HEN'S EGG. [CHAP. 



thrown into creases and wrinkles. It might almost be called 

 structureless, but under a high power a fine fibrillation is 

 visible, and a transverse section has a dotted or punctated 

 appearance ; it is probably therefore composed of fibres. 

 Its affinities are with elastic rather than connective tissue. 



The vitelline membrane of most vertebrates is perforated by fine pores. 

 These are largest in osseous fishes and much finer in mammals; they have not 

 been found in the vitelline membrane of birds. 



5. The whole space within the vitelline membrane is 

 occupied by the yolk. To the naked eye this appears toler- 

 ably uniform throughout, except at one particular point of 

 its surface, at which may be seen, lying immediately under 

 the vitelline membrane, a small white disc, about 4 mm. in 

 diameter. This is the blastoderm, or cicatricula. 



A tolerably typical cicatricula in a fecundated egg will 

 shew an outer white rim of some little breadth, and within 

 that a circular transparent area, in the centre of which, 

 again, there is an opacity, varying in appearance, sometimes 

 uniform, and sometimes dotted. 



The disc is always found to be uppermost whatever be 

 the position of the egg, provided there is no restraint to 

 the rotation of the yolk. The explanation of this is to be 

 sought for in the lighter specific gravity of that portion of 

 the yolk which is in the neighbourhood of the disc, and the 

 phenomenon is not in any way due to the action of the 

 chalazae. 



A section of the yolk of a hard-boiled egg will shew that 

 it is not perfectly uniform throughout, but that there is a 

 portion of it having the form of a flask, with a funnel- 

 shaped neck, which, when the egg is boiled, does not become 

 so solid as the rest of the yolk, but remains more or less fluid. 



The expanded neck of this flask-shaped space is situated 

 immediately underneath the disc, while its bulbous enlarge- 

 ment is about the middle of the yolk. We shall return to 

 it directly. 



6. The great mass of the yolk is composed of what is 

 known as the yellow yolk (Fig. 1, y. y.). This consists of 

 spheres (Fig. 2, A.} of from 25/4 to 100/x 1 in diameter, never 

 containing a nucleus, but filled with numerous minute highly 

 refractive granules ; these spheres are very delicate and easily 



= 'ooi mm. 



