66 



THE WAY LIFE BEGINS 



THE CHILD 



tubes. The instant the first 

 sperm cell gets through 

 the outer zone of the egg 

 a sheath or membrane is 

 thrown about the surface 

 of the egg which prevents 

 other sperm cells from 

 getting in. Occasionally, 

 however, it is believed, 

 two sperm get in. One 

 way of accounting for twins 

 that are physically united 

 like the Siamese twins is 

 to suppose that two sperm 

 cells have divided the egg 

 and the nuclear material 

 between them. United 

 bodies of this kind are 

 called 'identical twins'. 

 Twins are, of course, also 

 produced by the fertilizing 

 of two ova at the same 

 time, such being known as 

 'fraternal twins'. 



After the ovum has 

 united with a sperm cell it 

 is known as a fertilized cell, 

 whose hereditary substance 

 is derived from both par- 

 ents. The fertilized cell 

 of man and the mammals 

 cannot be distinguished, 

 and is very much like that 



THE RABBIT 



FIGURE g 

 The striking similarity in the appearance of the growing embryos of man and the mammals 

 is illustrated by comparing the human embryo of 21 days with the rabbit of a few days 

 (first pair of embryos above). There is, further, no essential difference between the human 

 embryo of 28 days and the rabbit of 7 or S days (second pair). After this time, however, the 

 difference becomes marked and increases as the rabbit approaches birth at 30 days and the 

 child at nine months (third pair). The child is of course larger at time of birth than the 

 rabbit (after Haeckel). 



