154 PRINCIPLES OF EMBRYOLOGY 



The significance of this movement is not yet fully understood (Nicholas 

 1945). It does not, however, affect the very thin coat which forms 

 the actual surface of the egg. This material, which has special 

 properties of elasticity and toughness, remains on the exterior surface 

 forming the boundary between the egg and the external medium, and 

 seems to play an important part in the biophysics of the morphological 

 changes which lead to the formation of the embryo (p. 439). 



The first sign of gastrulation is the appearance of a shallow groove, the 

 blastopore. This lies somewhat below the equator and within the area of 

 light-coloured yolky cells. As was said above, vital staining shows that it 

 appears in the region of the egg derived from the grey crescent, which by 

 this time is no longer recognisable. Before describing the later events, we 

 shall foUow the development of the bird embryo up to the corresponding 

 stage, so as to be in a position to compare the gastrulation of the two 

 forms. 



[b) The birds 



A discussion of the development of the bird's egg up to the blastula 

 stage will take much less space than was required for the amphibian. This 

 is not because the events are less complex, but because our knowledge of 

 them is less complete. The early stages of avian development remain 

 difficult to explore, partly because the enormous stores of yolk obscure 

 any cytoplasmic differentiation there may be, and partly because the egg 

 at this stage is out of easy reach within the body of its mother, who does 

 not lay it until the cleavage period is finished and the gastrulation begun. 



The true ovum of a bird such as the chick does not make up the whole 

 of what we usually call the egg, but only the so-called yolk. This is 

 covered with a well-defined and tough membrane — the vitelline mem- 

 brane — outside which lies the albumen or 'white' which is again enclosed 

 in a membrane, the whole being finally covered by the shell. All these 

 parts, from the vitelhne membrane outwards, are non-living additions 

 to the egg-cell, serving as sources of nourishment or means of protection ; 

 they are laid down around the ovum after it has left the ovary, been fertil- 

 ised, and is on its way down the oviduct. The 'yolk' or true egg-cell is 

 not adequately described by its popular name, since although it obviously 

 contains a very large quantity of yolk, this is by no means the whole or 

 the most important part of it. At one point (which lies in the plane of the 

 smallest section of the ovoid egg) there is a small area of clear cytoplasm 

 containing the nucleus. It is from this that the whole embryo is derived. 



The great stores of yolk affect development from the very beginning. 

 Whereas in most eggs, the penetration of one sperm suffices to prevent 



