THE HORMONES IN HUMAN REPRODUCTION 



through the canal of the cervix and body of the uterus and 

 into the oviduct. This journey is accomplished in a few hours, 

 so that the descending egg and the ascending sperm cells 

 meet in the Fallopian tube. There the process of fertilization 

 takes place by entry of one sperm cell into the egg, as de- 

 scribed for the sea urchin in the last chapter. 



The egg now begins to divide. The process of division has 

 not yet been observed in the human egg, but it has been well 

 studied in many animals by the drastic and expensive method 

 of killing animals at successive stages, a few hours apart, 

 after mating so that the eggs can be found and studied under 

 the microscope. In recent years the dividing eggs of rats, mice, 

 rabbits, and monkeys have, through the skill of Warren H. 

 Lewis of the Carnegie Embryological Laboratory and his 

 various associates, been successfully removed, kept in dishes 

 of salt solution at body temperature, and photographed in 

 motion pictures. Our illustration (Plate XI) is taken from 

 an excellent series of still photographs of the rabbit egg 

 taken by P. W. Gregory in the same laboratory. To those 

 who have not seen such pictures, this series will cause surprise 

 chiefly by its resemblance to the dividing sea urchin eggs of 

 Dr. Ethel Browne Harvey's series (Plate IV). 



In most mammals the embryos pass from the oviduct to 

 the uterus late on the third day or on the fourth. There is 

 some reason to think the same is true in the human. By this 

 time the embryos are at least in the four-cell stage and in some 

 animals (e.g. the rabbit) they have divided even more fully, 

 entering the uterus as little clumps of cells called morvlae 

 from the Latin word for mulberry^ which they resemble. The 



Plate XI. Division of the fertilized egg of the rabbit. A, one cell stage. 

 B, two cell stage, 25 hours after mating. C, four cells. D to G', 4 to 32 cells. 

 H, morula stage (solid mass of cells). /, first signs of hollowing. /, K, hollow 

 stages (blastocysts) 90 and 92 hours after mating. Magnified about 140 times. 

 From Contributions to Embryology, Carnegie Institution of "Washington, by 

 courtesy of P. W. Gregory. 



i 56 } 



