THE EGG 



resists such handling, but if a thin glass microscopic cover 

 slip is allowed to settle down upon an egg in a drop of water, 

 the clear membrane splits open under its pressure like the 

 skin of a grape and pops out its soft contents. It seems to 

 me quite remarkable that the infinitesimal sperm cell can 

 push its wa}' through this barrier. 



Some eggs, like those of the pig, are heavily laden with 

 fat globules (yolk) ; others, e.g. those of the rabbit, are so 

 clear that the nucleus, inevitable part of a living cell, can 

 be descried in the fresh egg. The human egg, as will be seen 

 from our photograph (Plate VII, D), is moderately filled 

 with yolk granules and the nucleus cannot be seen. If the 

 microscopist wishes to study details of the nucleus in any 

 species he must "fix" the egg, i.e. kill and harden it with 

 chemicals, and stain the nucleus with a dye solution. 



However different the eggs of birds and of mammals may 

 seem at first sight, they are fundamentally alike. Each is a 

 single cell, with a nucleus no bigger than that of many of the 

 ordinary cells of the body. The fact that the bird's egg is 

 very large for a single cell, and that even the mammal's egg 

 is the largest cell in the mammalian body, is due to the inclu- 

 sion of a considerable amount of stored food substance in the 

 cell. In the hen this yolk is enough to feed the chick until it 

 hatches ; in mammals it is only a few grains of fat and protein, 

 sufficient to provide energy for growth and cell division for 

 a few days, until the fertilized egg reaches the uterus. 



Since the bird's egg needs protection from harsh external 

 conditions — sunlight, dryness, a rough nest — it is provided 

 with a hard shell secreted about it by the oviduct after it 

 leaves the ovary. The mammalian egg, which is destined not 



Plate VIII. Stages in the development of the Graafian follicle of the rat. 

 Note the gradual enlargement of the cavity. In G the cells holding the egg to 

 the wall of the follicle have begun to degenerate, and the follicle is ready to 

 rupture. Magnified 60 times. From the Anatomical Record, by courtesy of J. L. 

 Boling and the Wistar Institute of Anatomy and Biology. 



{ 39 } 



