THE GENERAL SCHEME 



experiments by others, such fatherless young have actually 

 been raised to adult life, and to all appearances were normal 

 specimens of their kind. 



Lest this experiment should seem to disparage the im- 

 portance of the father, we should mention that the contrary 

 experiment also succeeds. If an egg is cut into two pieces, one 

 of which has no nucleus, and the latter is then entered by a 

 sperm cell, it too will divide and become an embryo, though 

 admittedly not as often as in the other, less drastic experi- 

 ment. In this case the embryo is motherless, from the stand- 

 point of heredity, for it has no egg nucleus in it. This shows 

 that egg stuff, to develop, must have a nucleus and requires 

 to be stimulated, but either an egg nucleus or a sperm nucleus 

 will do. We shall see later, however, that for reasons that 

 concern heredity it is decidedly better for the offspring to 

 get its nuclear material from both parents, as normally 

 happens. 



This remarkable experiment of artificial parthenogenesis 

 ("virgin generation") as it is called, has been repeated on 

 many kinds of animals, and it has been found that not only 

 magnesium solutions, but quite a number of different stimuli 

 will start division of the eggs. Exposure to sperm cells of 

 other species, extracts made from dead sperm cells, various 

 dilute acids and alkalies, sudden cooling, heating, shaking, or 

 pricking the eggs all can be used to initiate development in 

 one species or another. Mere staleness will cause the eggs of 

 some animals to divide. In all probability these diverse stimuli 

 produce some sort of common effect on the cell substances, 

 setting up internal changes (not as yet well understood) that 

 start the processes of division and growth of the egg. The 

 point of interest for us is that when the sperm cell acts upon 

 the egg in this way, it is exerting a merely physical or chem- 

 ical effect. The fact that it is itself a living cell is more or less 

 incidental. The egg contains all the essential elements for 



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