THE HORMONES IN HUMAN REPRODUCTION 



to leave a soft, moist, dark environment within its mother 

 until it is ready for birth, has no shell at all. The clear zone 

 that surrounds it is comparable with the shell membrane of 

 the hen's egg, familiar to everyone who has peeled the shell 

 off a boiled egg. The bird's egg also receives in the oviduct, 

 before the shell is laid on, a layer of albumen ("white of 

 egg") which no doubt helps to cushion the yolk and has some 

 nutritive value for the growing embryonic bird, but is chiefly 

 important because of its property of holding water and thus 

 preventing the egg from drying out by evaporation through 

 the shell. This is another protection the mammalian egg does 

 not require. It is therefore much smaller than the bird's egg, 

 because, in the first place, it lacks these massive provisions for 

 independent existence ; but also for a positive reason which my 

 mathematically minded readers may have perceived. The 

 mammalian egg, after only a few days of total dependence 

 upon its paltry yolk, gets its nourishment by absorption of 

 food and water by diffusion through its surface from the 

 uterine fluid in which it reposes. How much it gets depends 

 upon the area its surface presents to its surroundings. How 

 much it needs depends upon its volume. Geometry teaches 

 that as dimensions increase, surfaces increase as the square 

 of the radius, but volumes increase as the cube. As organic 

 bodies increase in size, therefore, the ratio between volume and 

 surface becomes less favorable. This rule, which has been 

 invoked to explain why animals do not grow to limitless 

 dimensions, probably operates also to keep all sorts of cells, 

 including eggs, within effective limits of size. 



The fate of unfertilized eggs. Not all the eggs formed in 

 the ovary, nor even a large proportion of them, go on to 

 reach fruition. Most domestic mammals shed eggs from the 

 ovary at regular intervals, the human one egg per month 

 approximately, the guinea pig four or five eggs every fifteen 

 days, the sow an average of about a dozen every twenty-one 

 days. If there is no mating in a given cycle, those eggs pro- 



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