REPRODUCTION 393 



We may safely conclude from what we find in the egg of 

 the star-fish and other forms that fertilization is promptly 

 followed by the cleavage .of the egg into two cells. In this 

 act of partition, as in others that come later, the nuclear 

 substance which seems so significant is divided with scrup- 

 ulous accuracy. 



Twins. It may happen that two ova are exposed to fer- 

 tilization at one time. If the double conception is accom- 

 plished, two embryos may find space for their development 

 in the uterus. The children that are born may be of op- 

 posite sexes and their resemblance may not be striking. 

 Twins that are "identical" being of the same sex and 

 wonderfully alike in the smallest features of their organiza- 

 tion probably have a different origin. These are believed 

 to have come from a single ovum. After the first cleavage 

 has taken place some mechanical influence has separated 

 the cells and marvellous to relate each one of them has 

 shown the power to form an entire human body. Identical 

 twins are said to be alike even to the lines on their finger- 

 tips. 



The first cleavage is soon followed by a second which 

 results in the formation of four cells. The plane of the 

 second division is at right angles to the first. A third 

 cleavage in a plane perpendicular to both the first and the 

 second gives rise to 8 cells. Additional cleavages convert 

 the cluster of cells into an aggregate suggestive of a berry 

 with its numerous lobules. Of course, until some new 

 substance is added, the embryo cannot exceed the size of 

 the ovum from which it has been evolved ; its cells must be- 

 come smaller and smaller as their number increases. 

 Sooner or later nourishment begins to be received from the 

 maternal blood and secretions. By degrees the placental 

 system described in Chapter XVII comes into existence. 

 But it remains true that, however much the bulk of the 

 new organism is enlarged, its every cell is a lineal descend- 

 ant of the fertilized egg and the two parents are equally 

 influential in shaping its destinies. 



The normal term of development (pregnancy or gestation] 



