After the cells of the first pair have undergone the next division, so that 

 there are now two groups of two cells each, we may again separate the 

 daughter cells. Each of these four can produce a complete individual, so 

 that perfect quadruplets result. In fact, the armadillo ordinarily gives birth 

 to four babies that appear to have been derived from the same egg. With 

 some species the cells of the third segmentation may be made to develop, 

 yielding eight identical individuals. There is reason to believe that all the 

 Dionne quintuplets came from the same egg (see illustration opposite). 



From what we know, it is reasonable to assume (1) that at some stage 

 in the growth of a mass of cells internal changes arise, and (2) that these, 

 in turn, influence the development of other parts. It has been shown, in 

 fact, that specific substances, or organizers, in various parts of the embryo 

 influence the development of other parts in such a way that all the parts 

 are kept related, or co-ordinated. There is evidence that relative positions 

 in the embryo also influence the development of tissues and organs. How- 

 ever, this may mean the same thing, namely, that particular substances, 

 produced in particular regions, influence the behavior of neighboring or 

 remoter cells in the course of development. 



How Can We Tell that There Are Specific Organizers 

 or Growth Substances? 



Position Is Everything — Almost In many games, in military opera- 

 tions, and in other human relations a great deal depends upon position. The 

 position of cells in an embryo seems also to be important. 



f'~^ 



After Stockard 



CHEMICAL MODIFICATION OF DEVELOPMENT 



360 



Ordinary minnow eggs develop 

 into familiar minnows, with one 

 eye on each side of the head. 

 Practically ail do so in ordinary 

 sea water, which contains many 

 different substances. By system- 

 atically changing the relative 

 amounts of magnesium and cal- 

 cium in the sea water, experi- 

 menters were able to make various 

 types of freak minnows hatch out 

 of the same batch of eggs. In one 

 very striking form that hatched in 

 water containing a high propor- 

 tion of magnesium the two eyes 

 started to develop on the right 

 and left sides but steadily moved 

 together and fused in the middle 



