STKUCTURE OF THE EGG. 7 



of birth has already attained a considerable size, and has pro- 

 ceeded nearly as far in its development as a chick at the time of 

 hatching. This is rendered possible by a special structure, the 

 placenta, by which the embryo rabbit is supplied through- 

 out its development with food, not from the egg itself, but 

 directly from the blood of the mother. 



Food-yolk is differently situated in the eggs of dififerent 

 animals. When only a small amount is present, it is fairly 

 uniformly distributed through the protoplasm of the egg-body. 

 Such eggs, as those of Amphioxus or of the rabbit, are called 

 alecithal. 



When food-yolk is more abundant, it usually accumulates 

 towards one pole of the egg, the opposite pole being compara- 

 tively free from yolk granules. Such eggs are called telolecithal ; 

 and in them, as in the frog's egg, development commences and 

 proceeds more rapidly at the pole in which there is least food- 

 yolk ; while in cases like the hen's egg, in which food-yolk is 

 extremely abundant and the egg is consequently of great size, 

 the developmental processes may be actually confined to this pole. 



In crabs and insects, and other members of the group of 

 Arthropods the food-yolk accumulates towards the centre of the 

 egg, the outermost layer of the egg, round its whole periphery, 

 remaining almost free from yolk granules. In such eggs, which 

 are called centrolecithal, development commences simultaneously 

 over the whole surface ; the central part of the egg, owing to 

 the hampering effect of the food-yolk, not taking part in the 

 developmental processes until a comparatively late stage. 



All eggs are devoid of yolk granules during the earlier stages 

 of their formation in the ovary. The yolk granules are usually 

 elaborated in special cells, which form capsules or follicles around 

 the eggs ; and the granules are passed from these follicular cells 

 into the interior of the eggs themselves. 



Maturation or Ripening of the Egg. 



After reaching its full size, and usually at or about the time 

 at which it leaves the ovary, but before the commencement of 

 actual development, the egg undergoes certain changes, which 

 are referred to as maturation or ripening ; and which may be 

 considered as a preparation on the part of the egg for fertilisa- 

 tion by the spermatozoon or male element. The changes in 



