THE HORMONES IN HUMAN REPRODUCTION 



the object of the whole process is to secure fusion of the 

 sperm cell with the egg nucleus. The tail of the sperm cell is 

 broken oif and left behind. The head now advances through 

 the eggf swelling slightly as it goes. In the egg substance a 

 star-shaped aster or region of stiffened egg substance ap- 

 pears and travels with the sperm nucleus. The egg nucleus 

 advances to meet the sperm and within ten minutes from the 

 time the sperm cell enters the egg, the two nuclei have united 

 and blended their substance (Plate III, C). The egg is now 

 fertilized; it inmiediately prepares to divide, and while the 

 observer watches, entranced with the smooth inevitability of 

 these events, the divisions follow one another every twenty-five 

 minutes, so that one cell becomes two, the two become four, 

 eight, sixteen, thirty-two, and so on until the embryo is a 

 mass of small cells looking like a mulberry. All these events 

 are shown in Plates III and IV. We need not follow the fer- 

 tilized egg through the subsequent complicated changes and 

 metamorphoses by which it becomes an adult sea urchin. 



The meaning of fertilization. If the eggs are left alone in 

 the dish, they do not go ahead by themselves and turn into 

 sea urchins. Development cannot begin until after the en- 

 trance of the sperm cell and the fusion of the nuclei. Evidently 

 the sperm cell in some way is necessary to set off or stimulate 

 division of the egg. A great step toward understanding what 

 happens was made by Jacques Loeb in 1899 and 1900. Loeb 

 had been studying the effect upon life processes of changing 

 the amounts of certain minerals which are present in living 

 tissue. By increasing or decreasing the concentration of mag- 

 nesium or calcium in sea water, for example, he could speed 

 up or slow down the rate of division of fertilized sea urchin 

 or starfish eggs. This led him to try dilute solutions of mag- 

 nesium chloride on the unfertilized eggs, completely free from 

 sperm cells. Eggs so treated began to divide and when care- 

 fully handled often went on to form complete larvae. In later 



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