ON PHYSIOLOGICAL MORPHOLOGY. 49 



imitate the stronger growth in the centre 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 

 imitate unequal growth, the centre 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 shown 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 His, the different regions 

 of the unsegmented ovum correspond already to the different 

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

 of preformed germ-regions gives no answer to the question, 

 Why do some parts of the embryo grow faster than others ? 

 Nevertheless, it is not necessarily in opposition to the theory 

 of growth that I offered in the preceding chapter. Starting 

 with the idea of His, we may well imagine that the different 

 regions of the ovum are somewhat different chemically, and 

 that these chemical differences of the different germ-regions 

 determine the differences of growth in the blastoderm. Thus 

 the phenomena of heteromorphosis would show that, in some 

 animals at least, the arrangement of preformed germ-regions 

 may be changed by gravitation, light, adhesion, etc. 



But, from the standpoint of causal morphology, it must be 

 asked what determines the arrangement of the different germ- 

 regions in the ovum. If we answer " heredity," causal mor- 

 phology can make no use of such an explanation. Our blood 

 has the temperature of about 37, but although our parents had 

 the same temperature its heat is not inherited, but is the result 

 of certain chemical processes in our tissues. Still it may be 

 possible that the molecular forces of the chemically different 

 substances of the ovum determine a separation of these 

 substances and at the same time the chief directions of the 

 future embryo. 



