100 The Mechanistic Conception of Life 



IV. THE artificial PRODUCTION OF DOUBLE AND MULTIPLE 



MONSTROSITIES IN SEA-URCHINS^ 



The idea that the formation of the vertebrate embryo is a 

 function of growth has been made the basis of the embryological 

 investigations of His. In a masterly way, His has sho^\^l how 

 inequality of growth determines the differentiation of organs. 

 In the blastoderm of a chick, for example, the first step in the 

 formation of the embryo is a process of folding. There origi- 

 nates a head fold, a tail fold, a medullary groove, and the system 

 of amniotic folds. According to His, all these processes of 

 folding are due simply to inequalities of growth, the center of 

 the blastoderm growing more rapidly than the periphery. It 

 can be shown, very simply, that such a process of unequal 

 growth must, indeed, lead to the formation of exactly such a 

 system of folds as we find in the blastoderm of a chick. If we 

 take a thin, flat plate of elastic rubber, and lay it on a drawing- 

 board, we can imitate the stronger growth in the center by 

 sticking two tacks into the middle of the rubber, a short 

 distance apart, and then pulling them in opposite directions. 

 In this way we may imitate unequal growth, the center growing 

 faster than the periphery. If we then fix the tacks in the 

 drawing-board, so that the rubber in the middle remains 

 stretched, we get the same system of folds as that sho\\Ti by the 

 embryo of a chick. I mention this way of demonstrating the 

 effects of unequal growth as the ideas of His are still doubted 

 by some morphologists. 



His raised the question. Why is growth different in different 

 parts of the blastoderm? But instead of trying to answer it 

 from the physiological standpoint he answered it from the 

 anatomical standpoint. According to him, the different regions 

 of the unsegmented egg correspond alread}' to the different 

 regions of the differentiated embryo. But this so-called theory 



1 Another method of producing twins from one egg is discussed in the last 

 chapter of this book. 



